Refactor GDScript/C# script templates logic to be editor-only

Not a full refactor as it still goes through ScriptLanguage so it's hacky,
but at least it can now compile without this.

modules/mono/editor/script_templates/Object/empty.cs

@@ -0,0 +1,8 @@
+// meta-description: Empty template suitable for all Objects
+
+using _BINDINGS_NAMESPACE_;
+using System;
+
+public partial class _CLASS_ : _BASE_
+{
+}