docs: add license header to ArgumentsHandler.cs

Author: RobynLlama

src/SharpWebserver/ListenServer/ArgumentsHandler.cs

@@ -1,3 +1,12 @@
+/*
+Copyright (C) 2025 Robyn (robyn@mamallama.dev)
+
+    This program is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+*/
+
 using System;
 using System.Collections.Generic;