Medusa System Added

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

src/NadekoBot/Common/ModuleBehaviors/IExecOnMessage.cs

@@ -0,0 +1,21 @@
+namespace NadekoBot.Common.ModuleBehaviors;
+
+/// <summary>
+///     Implemented by modules to handle non-bot messages received
+/// </summary>
+public interface IExecOnMessage
+{
+    int Priority { get; }
+
+    /// <summary>
+    /// Ran after a non-bot message was received
+    /// *<see cref="IExecOnMessage"/>* →
+    /// <see cref="IInputTransformer"/> →
+    /// <see cref="IExecPreCommand"/> →
+    /// [<see cref="IExecPostCommand"/> | <see cref="IExecNoCommand"/>]
+    /// </summary>
+    /// <param name="guild">Guild where the message was sent</param>
+    /// <param name="msg">The message that was received</param>
+    /// <returns>Whether further processing of this message should be blocked</returns>
+    Task<bool> ExecOnMessageAsync(IGuild guild, IUserMessage msg);
+}