Update to Godot 4

[OverloadedOrama]

Before merging this PR, I strongly suggest creating a 3.x branch, so the 3.x code can still be available for anyone that might need it.

This PR also changes the code to make better use of static typing, such as using classes instead of dictionaries. Statically typed GDScript results in increased performance in Godot 4. I did a small benchmark with the example spritesheet, by taking the average of 10 spritesheet slices. The performance increase is not that huge for the example spritesheet, but it should be noticeable with larger images.

3.x	4.x with no code changes	4.x with better static typing
233 ms	182 ms	151 ms

Known limitations:
The compatibility renderer does not yet work with multi-threaded mode (godotengine/godot#61650). Multi-threaded mode is marked as experimental in general, so use it with care, until Godot fixes the issues with it.

[Variable-ind]

Thanks