Fix crash when deleting paragraph ending \n in Firefox

[12joan]

Description
Due to a Firefox browser quirk, the DOM selection sometimes includes the additional \n added when isTrailing is true in TextString. This results in an invalid focus point, causing a crash during delete operations.

Issue
Fixes: #5273

Example
Before:

before.mp4

After:

after.mp4

Context
It's possible that the logic for the check I added might not be perfect. However, I'm 99% sure that any new bugs it introduces will be harder to reproduce accidentally than the bug it fixes.

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: 54bc0f2

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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[dylans]

GitHub actions is providing a linting warning that should be addressed. Please review that and then we can land this.

[12joan]

Hi @dylans, I can't see the linting warning you're referring to, unless you mean this warning that's also present on main?

/Users/joe/workspace/slate/packages/slate-react/src/components/editable.tsx
  193:6  warning  React Hook useLayoutEffect has a missing dependency: 'state'. Either include it or remove the dependency array  react-hooks/exhaustive-deps

✖ 1 problem (0 errors, 1 warning)

If that's the warning you mean, perhaps it should be fixed in a separate PR?