Fix backspacing with checkbox issue

[OliverWales]

Backspacing at the start of a text node that's after an empty checkbox deletes the checkbox rather than deleting the newline and putting the text in the checkbox nod. Same applies to bulleted list items.

Screen.Recording.2023-09-01.at.15.11.24.mov

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[zbeyens]

Closing it as it should be fixed in Slate