[refactor] Extract BuiltinCommands.cpp from PlayerManager.cpp

And move itemGMGiveHandler() from ItemManager.

src/BuiltinCommands.hpp

@@ -0,0 +1,9 @@
+#pragma once
+
+#include "CNProtocol.hpp"
+
+namespace BuiltinCommands {
+    void init();
+
+    void setSpecialState(CNSocket *sock, CNPacketData *data);
+};