Add a stop feature to BlockNoteAI

[virgile-dev]

Is your feature request related to a problem? Please describe.
Once the AI is thinking no way to stop it.

Describe the solution you'd like
Add a button stop in the AI toolbar

Additional context
Here is our custom design for Docs that you can reuse

Bonus
[x] I'm a sponsor and would appreciate if you could look into this sooner than later 💖

[YousefED]

Thanks, will prioritize this!

[woutervroege]

Linking #1806