Autoformat undo

[dylans]

The enableUndoOnDelete flag was recently added to the autoformat plugin which is a much needed addition. There are a few challenges from my perspective:

• character to press to cancel the autoformat should be configurable, for example maybe you want both backspace and escape to work
• the logic to undo the autoformatting does not seem to work in a collaborative editing environment (e.g. when using slate-yjs). Pressing backspace/delete is simply deleting the entire autoformatted section including the symbol. My hunch is that we need a way to go back one step in history rather than trying to replace things with the symbol, but I could be wrong on this point.
• the implementation only works for text rules, not marks or other autoformat rules...