Enable global classes in subfolders of res://addons/

[emklasson]

This PR lets GlobalClass classes get added to the "Create New Node" dialog no matter where they are under addons/ as long as it's not in a disabled plugin (or any subfolder of such as those are assumed to belong to the disabled plugin).

This also then lets you keep non-plugin files under addons/ and have their GlobalClasses work, as seems encouraged by https://docs.godotengine.org/en/stable/tutorials/best_practices/project_organization.html saying "In general, keep third-party resources in a top-level addons/ folder, even if they aren't editor plugins. This makes it easier to track which files are third-party."

Fixes #78400.

The code could presumably be optimised some e.g. by not using String::split, but I'm guessing code clarity and brevity takes preference over squeezing out performance here?

[ajreckof]

LGTM.
I don't see how to do it better then with split and based on what is done in EditorPluginSettings::_get_plugins it should be good as it is.

[akien-mga]

Superseded by #91337. Thanks for the contribution!