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/EditorScript/basic_editor_script.cs

@@ -0,0 +1,15 @@
+// meta-description: Basic editor script template
+
+#if TOOLS
+using _BINDINGS_NAMESPACE_;
+using System;
+
+[Tool]
+public partial class _CLASS_ : _BASE_
+{
+    // Called when the script is executed (using File -> Run in Script Editor).
+    public override void _Run()
+    {
+    }
+}
+#endif