Fix onDragStart callback dependency on 'readonly'

[ahoisl]

Description
PR #4617 fixed a bug by checking readOnly in the onDragStart callback, but missed setting the dependency on useCallback. Consequently, if you have an editable that switches from readonly to not-readonly, the drag event will not add the internal fragment and the drop will fallback to plain text (because the fragment is not there).

Issue
related to #4548

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: 72c2dbc

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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