Medusa System Added

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

src/NadekoBot/Common/ModuleBehaviors/IExecNoCommand.cs

@@ -0,0 +1,19 @@
+namespace NadekoBot.Common.ModuleBehaviors;
+
+/// <summary>
+///     Executed if no command was found for this message
+/// </summary>
+public interface IExecNoCommand
+{
+    /// <summary>
+    /// Executed at the end of the lifecycle if no command was found
+    /// <see cref="IExecOnMessage"/> →
+    /// <see cref="IInputTransformer"/> →
+    /// <see cref="IExecPreCommand"/> →
+    /// [<see cref="IExecPostCommand"/> | *<see cref="IExecNoCommand"/>*]
+    /// </summary>
+    /// <param name="guild"></param>
+    /// <param name="msg"></param>
+    /// <returns>A task representing completion</returns>
+    Task ExecOnNoCommandAsync(IGuild guild, IUserMessage msg);
+}