Fix global script class parsing.

* Broke with #72226
* Restored previous version of the code, made it even more error tolerant.
* Added a warning to **not** change the code.

Fixes #72226.

modules/gdscript/tests/scripts/parser/errors/class_name_after_annotation.gd

@@ -1,5 +1,5 @@
 # Error here. Annotations should be used before `class_name`, not after.
-class_name HelloWorld
+class_name WrongAnnotationPlace
 @icon("res://path/to/optional/icon.svg")
 
 func test():