Fix crash on drag and drop image on readOnly editable

[imdbsd]

Description
when readonly, onDragStart function should not be called since it will mutate the document.

Issue
Fixes: #4548

Example
https://user-images.githubusercontent.com/16449020/138408160-c7ce17db-65cf-4d9b-9f55-c8d907b96bac.mp4

Context

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: f3fedb9

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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[ahoisl]

This fix causes DnD to not insert fragments any more after switching an Editable from readonly to not-readonly

Reason: the onDragStart useCallback dependencies are missing the "readonly" property. I will open a PR soon...