Medusa System Added

Read about the medusa system [here](https://nadekobot.readthedocs.io/en/latest/medusa/creating-a-medusa/)

src/Nadeko.Medusa/Strings/StringsLoader.cs

@@ -0,0 +1,137 @@
+using System.Diagnostics.CodeAnalysis;
+using Serilog;
+using YamlDotNet.Serialization;
+
+namespace Nadeko.Snake;
+
+/// <summary>
+///     Loads strings from the shortcut or localizable path
+/// </summary>
+public class StringsLoader
+{
+    private readonly string _localizableResponsesPath;
+    private readonly string _shortcutResponsesFile;
+    
+    private readonly string _localizableCommandsPath;
+    private readonly string _shortcutCommandsFile;
+
+    public StringsLoader(string basePath)
+    {
+        _localizableResponsesPath = Path.Join(basePath, "strings/res");
+        _shortcutResponsesFile = Path.Join(basePath, "res.yml");
+        
+        _localizableCommandsPath = Path.Join(basePath, "strings/cmds");
+        _shortcutCommandsFile = Path.Join(basePath, "cmds.yml");
+    }
+
+    public IReadOnlyDictionary<string, IReadOnlyDictionary<string, CommandStrings>> GetCommandStrings()
+    {
+        var outputDict = new Dictionary<string, IReadOnlyDictionary<string, CommandStrings>>();
+
+        if (File.Exists(_shortcutCommandsFile))
+        {
+            if (TryLoadCommandsFromFile(_shortcutCommandsFile, out var dict, out _))
+            {
+                outputDict["en-us"] = dict;
+            }
+
+            return outputDict;
+        }
+
+        if (Directory.Exists(_localizableCommandsPath))
+        {
+            foreach (var cmdsFile in Directory.EnumerateFiles(_localizableCommandsPath))
+            {
+                if (TryLoadCommandsFromFile(cmdsFile, out var dict, out var locale) && locale is not null)
+                {
+                    outputDict[locale.ToLowerInvariant()] = dict;
+                }
+            }
+        }
+
+        return outputDict;
+    }
+
+    
+    private static readonly IDeserializer _deserializer = new DeserializerBuilder().Build();
+    private static bool TryLoadCommandsFromFile(string file,
+        [NotNullWhen(true)] out IReadOnlyDictionary<string, CommandStrings>? strings,
+        out string? localeName)
+    {
+        try
+        {
+            var text = File.ReadAllText(file);
+            strings = _deserializer.Deserialize<Dictionary<string, CommandStrings>?>(text)
+                      ?? new();
+            localeName = GetLocaleName(file);
+            return true;
+        }
+        catch (Exception ex)
+        {
+            Log.Error(ex, "Error loading {FileName} command strings: {ErrorMessage}", file, ex.Message);
+        }
+
+        strings = null;
+        localeName = null;
+        return false;
+    }
+
+
+    public IReadOnlyDictionary<string, IReadOnlyDictionary<string, string>> GetResponseStrings()
+    {
+        var outputDict = new Dictionary<string, IReadOnlyDictionary<string, string>>();
+
+        // try to load a shortcut file
+        if (File.Exists(_shortcutResponsesFile))
+        {
+            if (TryLoadResponsesFromFile(_shortcutResponsesFile, out var dict, out _))
+            {
+                outputDict["en-us"] = dict;
+            }
+
+            return outputDict;
+        }
+
+        if (!Directory.Exists(_localizableResponsesPath))
+            return outputDict;
+        
+        // if shortcut file doesn't exist, try to load localizable files
+        foreach (var file in Directory.GetFiles(_localizableResponsesPath))
+        {
+            if (TryLoadResponsesFromFile(file, out var strings, out var localeName) && localeName is not null)
+            {
+                outputDict[localeName.ToLowerInvariant()] = strings;
+            }
+        }
+
+        return outputDict;
+    }
+
+    private static bool TryLoadResponsesFromFile(string file,
+        [NotNullWhen(true)] out IReadOnlyDictionary<string, string>? strings,
+        out string? localeName)
+    {
+        try
+        {
+            strings = _deserializer.Deserialize<Dictionary<string, string>?>(File.ReadAllText(file));
+            if (strings is null)
+            {
+                localeName = null;
+                return false;
+            }
+
+            localeName = GetLocaleName(file).ToLowerInvariant();
+            return true;
+        }
+        catch (Exception ex)
+        {
+            Log.Error(ex, "Error loading {FileName} response strings: {ErrorMessage}", file, ex.Message);
+            strings = null;
+            localeName = null;
+            return false;
+        }
+    }
+
+    private static string GetLocaleName(string fileName)
+        => Path.GetFileNameWithoutExtension(fileName);
+}