Restructured the project structure back to the way it was, there's no reasonable way to split the modules

src/NadekoBot/_common/Yml/CommentAttribute.cs

@@ -0,0 +1,11 @@
+#nullable disable
+namespace NadekoBot.Common.Yml;
+
+[AttributeUsage(AttributeTargets.Property)]
+public class CommentAttribute : Attribute
+{
+    public string Comment { get; }
+
+    public CommentAttribute(string comment)
+        => Comment = comment;
+}

src/NadekoBot/_common/Yml/CommentGatheringTypeInspector.cs

@@ -0,0 +1,65 @@
+#nullable disable
+using YamlDotNet.Core;
+using YamlDotNet.Serialization;
+using YamlDotNet.Serialization.TypeInspectors;
+
+namespace NadekoBot.Common.Yml;
+
+public class CommentGatheringTypeInspector : TypeInspectorSkeleton
+{
+    private readonly ITypeInspector _innerTypeDescriptor;
+
+    public CommentGatheringTypeInspector(ITypeInspector innerTypeDescriptor)
+        => _innerTypeDescriptor = innerTypeDescriptor ?? throw new ArgumentNullException(nameof(innerTypeDescriptor));
+
+    public override IEnumerable<IPropertyDescriptor> GetProperties(Type type, object container)
+        => _innerTypeDescriptor.GetProperties(type, container).Select(d => new CommentsPropertyDescriptor(d));
+
+    private sealed class CommentsPropertyDescriptor : IPropertyDescriptor
+    {
+        public string Name { get; }
+
+        public Type Type
+            => _baseDescriptor.Type;
+
+        public Type TypeOverride
+        {
+            get => _baseDescriptor.TypeOverride;
+            set => _baseDescriptor.TypeOverride = value;
+        }
+
+        public int Order { get; set; }
+
+        public ScalarStyle ScalarStyle
+        {
+            get => _baseDescriptor.ScalarStyle;
+            set => _baseDescriptor.ScalarStyle = value;
+        }
+
+        public bool CanWrite
+            => _baseDescriptor.CanWrite;
+
+        private readonly IPropertyDescriptor _baseDescriptor;
+
+        public CommentsPropertyDescriptor(IPropertyDescriptor baseDescriptor)
+        {
+            _baseDescriptor = baseDescriptor;
+            Name = baseDescriptor.Name;
+        }
+
+        public void Write(object target, object value)
+            => _baseDescriptor.Write(target, value);
+
+        public T GetCustomAttribute<T>()
+            where T : Attribute
+            => _baseDescriptor.GetCustomAttribute<T>();
+
+        public IObjectDescriptor Read(object target)
+        {
+            var comment = _baseDescriptor.GetCustomAttribute<CommentAttribute>();
+            return comment is not null
+                ? new CommentsObjectDescriptor(_baseDescriptor.Read(target), comment.Comment)
+                : _baseDescriptor.Read(target);
+        }
+    }
+}

src/NadekoBot/_common/Yml/CommentsObjectDescriptor.cs

@@ -0,0 +1,30 @@
+#nullable disable
+using YamlDotNet.Core;
+using YamlDotNet.Serialization;
+
+namespace NadekoBot.Common.Yml;
+
+public sealed class CommentsObjectDescriptor : IObjectDescriptor
+{
+    public string Comment { get; }
+
+    public object Value
+        => _innerDescriptor.Value;
+
+    public Type Type
+        => _innerDescriptor.Type;
+
+    public Type StaticType
+        => _innerDescriptor.StaticType;
+
+    public ScalarStyle ScalarStyle
+        => _innerDescriptor.ScalarStyle;
+
+    private readonly IObjectDescriptor _innerDescriptor;
+
+    public CommentsObjectDescriptor(IObjectDescriptor innerDescriptor, string comment)
+    {
+        _innerDescriptor = innerDescriptor;
+        Comment = comment;
+    }
+}

src/NadekoBot/_common/Yml/CommentsObjectGraphVisitor.cs

@@ -0,0 +1,29 @@
+#nullable disable
+using YamlDotNet.Core;
+using YamlDotNet.Core.Events;
+using YamlDotNet.Serialization;
+using YamlDotNet.Serialization.ObjectGraphVisitors;
+
+namespace NadekoBot.Common.Yml;
+
+public class CommentsObjectGraphVisitor : ChainedObjectGraphVisitor
+{
+    public CommentsObjectGraphVisitor(IObjectGraphVisitor<IEmitter> nextVisitor)
+        : base(nextVisitor)
+    {
+    }
+
+    public override bool EnterMapping(IPropertyDescriptor key, IObjectDescriptor value, IEmitter context)
+    {
+        if (value is CommentsObjectDescriptor commentsDescriptor
+            && !string.IsNullOrWhiteSpace(commentsDescriptor.Comment))
+        {
+            var parts = commentsDescriptor.Comment.Split('\n');
+
+            foreach (var part in parts)
+                context.Emit(new Comment(part.Trim(), false));
+        }
+
+        return base.EnterMapping(key, value, context);
+    }
+}

src/NadekoBot/_common/Yml/MultilineScalarFlowStyleEmitter.cs

@@ -0,0 +1,35 @@
+#nullable disable
+using YamlDotNet.Core;
+using YamlDotNet.Serialization;
+using YamlDotNet.Serialization.EventEmitters;
+
+namespace NadekoBot.Common.Yml;
+
+public class MultilineScalarFlowStyleEmitter : ChainedEventEmitter
+{
+    public MultilineScalarFlowStyleEmitter(IEventEmitter nextEmitter)
+        : base(nextEmitter)
+    {
+    }
+
+    public override void Emit(ScalarEventInfo eventInfo, IEmitter emitter)
+    {
+        if (typeof(string).IsAssignableFrom(eventInfo.Source.Type))
+        {
+            var value = eventInfo.Source.Value as string;
+            if (!string.IsNullOrEmpty(value))
+            {
+                var isMultiLine = value.IndexOfAny(new[] { '\r', '\n', '\x85', '\x2028', '\x2029' }) >= 0;
+                if (isMultiLine)
+                {
+                    eventInfo = new(eventInfo.Source)
+                    {
+                        Style = ScalarStyle.Literal
+                    };
+                }
+            }
+        }
+
+        nextEmitter.Emit(eventInfo, emitter);
+    }
+}

src/NadekoBot/_common/Yml/Rgba32Converter.cs

@@ -0,0 +1,47 @@
+#nullable disable
+using SixLabors.ImageSharp.PixelFormats;
+using System.Globalization;
+using YamlDotNet.Core;
+using YamlDotNet.Core.Events;
+using YamlDotNet.Serialization;
+
+namespace NadekoBot.Common.Yml;
+
+public class Rgba32Converter : IYamlTypeConverter
+{
+    public bool Accepts(Type type)
+        => type == typeof(Rgba32);
+
+    public object ReadYaml(IParser parser, Type type)
+    {
+        var scalar = parser.Consume<Scalar>();
+        var result = Rgba32.ParseHex(scalar.Value);
+        return result;
+    }
+
+    public void WriteYaml(IEmitter emitter, object value, Type type)
+    {
+        var color = (Rgba32)value;
+        var val = (uint)((color.B << 0) | (color.G << 8) | (color.R << 16));
+        emitter.Emit(new Scalar(val.ToString("X6").ToLower()));
+    }
+}
+
+public class CultureInfoConverter : IYamlTypeConverter
+{
+    public bool Accepts(Type type)
+        => type == typeof(CultureInfo);
+
+    public object ReadYaml(IParser parser, Type type)
+    {
+        var scalar = parser.Consume<Scalar>();
+        var result = new CultureInfo(scalar.Value);
+        return result;
+    }
+
+    public void WriteYaml(IEmitter emitter, object value, Type type)
+    {
+        var ci = (CultureInfo)value;
+        emitter.Emit(new Scalar(ci.Name));
+    }
+}

src/NadekoBot/_common/Yml/UriConverter.cs

@@ -0,0 +1,25 @@
+#nullable disable
+using YamlDotNet.Core;
+using YamlDotNet.Core.Events;
+using YamlDotNet.Serialization;
+
+namespace NadekoBot.Common.Yml;
+
+public class UriConverter : IYamlTypeConverter
+{
+    public bool Accepts(Type type)
+        => type == typeof(Uri);
+
+    public object ReadYaml(IParser parser, Type type)
+    {
+        var scalar = parser.Consume<Scalar>();
+        var result = new Uri(scalar.Value);
+        return result;
+    }
+
+    public void WriteYaml(IEmitter emitter, object value, Type type)
+    {
+        var uri = (Uri)value;
+        emitter.Emit(new Scalar(uri.ToString()));
+    }
+}

src/NadekoBot/_common/Yml/Yaml.cs

@@ -0,0 +1,28 @@
+#nullable disable
+using YamlDotNet.Serialization;
+using YamlDotNet.Serialization.NamingConventions;
+
+namespace NadekoBot.Common.Yml;
+
+public class Yaml
+{
+    public static ISerializer Serializer
+        => new SerializerBuilder().WithTypeInspector(inner => new CommentGatheringTypeInspector(inner))
+                                  .WithEmissionPhaseObjectGraphVisitor(args
+                                      => new CommentsObjectGraphVisitor(args.InnerVisitor))
+                                  .WithEventEmitter(args => new MultilineScalarFlowStyleEmitter(args))
+                                  .WithNamingConvention(CamelCaseNamingConvention.Instance)
+                                  .WithIndentedSequences()
+                                  .WithTypeConverter(new Rgba32Converter())
+                                  .WithTypeConverter(new CultureInfoConverter())
+                                  .WithTypeConverter(new UriConverter())
+                                  .Build();
+
+    public static IDeserializer Deserializer
+        => new DeserializerBuilder().WithNamingConvention(CamelCaseNamingConvention.Instance)
+                                    .WithTypeConverter(new Rgba32Converter())
+                                    .WithTypeConverter(new CultureInfoConverter())
+                                    .WithTypeConverter(new UriConverter())
+                                    .IgnoreUnmatchedProperties()
+                                    .Build();
+}