Medusa System Added

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

src/Nadeko.Medusa/Attributes/cmdAttribute.cs

@@ -0,0 +1,37 @@
+namespace Nadeko.Snake;
+
+/// <summary>
+/// Marks a method as a snek command
+/// </summary>
+[AttributeUsage(AttributeTargets.Method)]
+public class cmdAttribute : Attribute
+{
+    /// <summary>
+    /// Command description. Avoid using, as cmds.yml is preferred
+    /// </summary>
+    public string? desc { get; set; }
+    
+    /// <summary>
+    /// Command args examples. Avoid using, as cmds.yml is preferred
+    /// </summary>
+    public string[]? args { get; set; }
+    
+    /// <summary>
+    /// Command aliases
+    /// </summary>
+    public string[] Aliases { get; }
+
+    public cmdAttribute()
+    {
+        desc = null;
+        args = null;
+        Aliases = Array.Empty<string>();
+    }
+    
+    public cmdAttribute(params string[] aliases)
+    {
+        Aliases = aliases;
+        desc = null;
+        args = null;
+    }
+}