Fix performance issue with renderPlaceholder

[12joan]

Description
PR #5310 introduced a performance issue when typing in large documents, caused by adding renderPlaceholder to useMemo in element.tsx. This PR fixes that performance issue by wrapping the default renderPlaceholder in a useCallback.

In addition, all custom renderPlaceholder functions passed to Editable should also be wrapped in useCallback.

Issue
Fixes: #5335

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: 33b6352

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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