Fix contenteditalbe firefox table selection

[WcaleNieWolny]

Description
This PR fixes the contenteditable table selection in firefox. Firefox uses a custom editor when table is contenteditable and this adds support for this custom editor.

Issue
Fixes: None, but it does fix this

Example

2023-07-30.08-34-11.mp4

Before this PR it threw some exception IIRC

Context
Firefox basically uses rows in the contenteditable table selection and startOffset is the cell offset. By walking down the DOM tree we can get the sames values as in chrome. I also added some checks to make sure this does not crash.

Checks

• The new code matches the existing patterns and styles.
• The tests pass with yarn test.
• The linter passes with yarn lint. (Fix errors with yarn fix.)
• The relevant examples still work. (Run examples with yarn start.)
• You've added a changeset if changing functionality. (Add one with yarn changeset add.)

[changeset-bot]

🦋 Changeset detected

Latest commit: 55b3b60

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
slate-react	Patch

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