Improve delete key behavior near image blocks

[jorgefreires]

Is your feature request related to a problem? Please describe.

I experience unexpected behavior when working with images in BlockNote. When editing a paragraph adjacent to an image block and pressing the Delete key at the start of the subsequent line, the image gets deleted rather than the cursor moving into the image block.

Examples of current behavior vs expected behavior:

Expected (Google Docs behavior):
https://github.com/user-attachments/assets/1e1f97f8-2438-4d63-8b23-36a991ea8711
Cursor moves into image block when deleting at line start

Current (BlockNote behavior):
https://github.com/user-attachments/assets/f0b24e02-7ff8-479e-a75d-e05328059d20
Image gets deleted when using Delete key at line start

Describe the solution you'd like
The cursor should move into the image block when pressing Delete at the beginning of the subsequent line, rather than deleting the image. This should mirror the behavior seen in Google Docs and other mainstream text editors.