Variant Constants

[bonjorno7]

While refactoring Color conversions and constants into a module I ran into the issue that classes can have integer constants, but only Variant can have Variant constants.
This PR implements the ability the add Variant constants to any class: godotengine/godot#77293
It affects a lot of the engine, so I'd like to have a discussion about what needs to be done.

[bonjorno7]

I could also replace integer constants with variant constants entirely, though that would complicate things with enums.