Change bulk_theme_override variable to an integer

[peachey2k2]

TL;DR: The value being a boolean caused issues with editor plugins that modified active themes. Even when the user calls begin_bulk_theme_override() themselves, engine would internally call begin_bulk_theme_override() and end_bulk_theme_override(), essentially overwriting the previous override call.

Related proposal

Bugsquad edit: Closes godotengine/godot-proposals#9672

[KoBeWi]

The new behavior needs to be noted in the documentation of bulk_theme_override methods.

EDIT:
While this is a simple change, it does not come without consequences. If someone had a code where bulk operation begins multiple times without ending it, now it will stop working without any error. idk if it's something worth being concerned about. It would've been easier if begin_bulk_theme_override() had resulted in an error when the operation was already started; the risk of breaking code now would be non-existent.