Medusa System Added

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

src/NadekoBot/Common/Medusa/Adapters/BehaviorAdapter.cs

@@ -0,0 +1,76 @@
+#nullable enable
+
+[DontAddToIocContainer]
+public sealed class BehaviorAdapter : ICustomBehavior
+{
+    private readonly WeakReference<Snek> _snekWr;
+    private readonly IMedusaStrings _strings;
+    private readonly IServiceProvider _services;
+    private readonly string _name;
+
+    // unused
+    public int Priority
+        => 0;
+
+    public BehaviorAdapter(WeakReference<Snek> snekWr, IMedusaStrings strings, IServiceProvider services)
+    {
+        _snekWr = snekWr;
+        _strings = strings;
+        _services = services;
+
+        _name = snekWr.TryGetTarget(out var snek)
+            ? $"snek/{snek.GetType().Name}"
+            : "unknown";
+    }
+
+    public async Task<bool> ExecPreCommandAsync(ICommandContext context, string moduleName, CommandInfo command)
+    {
+        if (!_snekWr.TryGetTarget(out var snek))
+            return false;
+
+        return await snek.ExecPreCommandAsync(ContextAdapterFactory.CreateNew(context, _strings, _services),
+            moduleName,
+            command.Name);
+    }
+
+    public async Task<bool> ExecOnMessageAsync(IGuild? guild, IUserMessage msg)
+    {
+        if (!_snekWr.TryGetTarget(out var snek))
+            return false;
+
+        return await snek.ExecOnMessageAsync(guild, msg);
+    }
+
+    public async Task<string?> TransformInput(
+        IGuild guild,
+        IMessageChannel channel,
+        IUser user,
+        string input)
+    {
+        if (!_snekWr.TryGetTarget(out var snek))
+            return null;
+        
+        return await snek.ExecInputTransformAsync(guild, channel, user, input);
+    }
+
+    public async Task ExecOnNoCommandAsync(IGuild? guild, IUserMessage msg)
+    {
+        if (!_snekWr.TryGetTarget(out var snek))
+            return;
+
+        await snek.ExecOnNoCommandAsync(guild, msg);
+    }
+
+    public async ValueTask ExecPostCommandAsync(ICommandContext context, string moduleName, string commandName)
+    {
+        if (!_snekWr.TryGetTarget(out var snek))
+            return;
+        
+        await snek.ExecPostCommandAsync(ContextAdapterFactory.CreateNew(context, _strings, _services),
+            moduleName,
+            commandName);
+    }
+
+    public override string ToString()
+        => _name;
+}