Support custom autoformatting rules

[danielrearden]

Is your feature request related to a problem? Please describe.
It's common to want to implement certain behavior after a user types a combination of characters. Examples from our own app:

• ``` - creates a codeblock
• > - creates a blockquote
• --- - creates a divider

Describe the solution you'd like
It would be nice if Blocknote provided a way to set these sort of autoformatting rules, at least at the block level. Not all blocks need them, but some would definitely benefit from having this as an option

Describe alternatives you've considered
We've worked around this by listening to changes in the content and then mutating the content accordingly when one of the rules is triggered, but that's not ideal.

Additional context
Add any other context or screenshots about the feature request here.

Bonus

• I'm a sponsor and would appreciate if you could look into this sooner than later 💖