Implements #5205, I'm not 100% convinced by the API names, alternatively we can use something like undoStatesCount or call them undoSteps (or stepCount)

I think undoHistory.undoSteps and undoHistory.redoSteps might be a good option.

In a future we could have undoHistory.undoStates (or undoHistory.states) for a future API to get/move to a specific position/state in the undo history. But that requires several new objects to make it useful.

I also figured Site was the place to put this but it could be moved without much issue as long as we have document access, it makes sense to me since it's a level above the sprite and scripting has no concept of a "document".

Site is like a snapshot of the editor/timeline position in a specific time (app.site the current site), it's not updated when it's stored. Actually the best position for the undoHistory is the Sprite. So we can do sprite.undoHistory.

In some way the internal app::Doc + doc::Sprite is represented by the scripting Sprite (check sprite_class.cpp functions to see how the document is obtained from the sprite).

Renamed and moved to Sprite

Looks great! 👍