Medusa System Added

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

src/NadekoBot/Common/ModuleBehaviors/IExecPostCommand.cs

@@ -0,0 +1,22 @@
+namespace NadekoBot.Common.ModuleBehaviors;
+
+/// <summary>
+/// This interface's method is executed after the command successfully finished execution.
+/// ***There is no support for this method in NadekoBot services.***
+/// It is only meant to be used in medusa system
+/// </summary>
+public interface IExecPostCommand
+{
+    /// <summary>
+    /// Executed after a command was successfully executed
+    /// <see cref="IExecOnMessage"/> →
+    /// <see cref="IInputTransformer"/> →
+    /// <see cref="IExecPreCommand"/> →
+    /// [*<see cref="IExecPostCommand"/>* | <see cref="IExecNoCommand"/>]
+    /// </summary>
+    /// <param name="ctx">Command context</param>
+    /// <param name="moduleName">Module name</param>
+    /// <param name="commandName">Command name</param>
+    /// <returns>A task representing completion</returns>
+    ValueTask ExecPostCommandAsync(ICommandContext ctx, string moduleName, string commandName);
+}