Fix error when triple-clicking a word preceding a contenteditable="false" DOM node in Chrome

[12joan]

Description
Triple-clicking a word in Chrome will sometimes place the focus inside a contenteditable="false" DOM node following the word, which causes toSlatePoint to throw an error. This PR checks for this and adjusts the focusNode if necessary.

Issue
Fixes #5341

Context
#4329 (comment)

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.)