Fix default focus styles

[Chudesnov]

Resolves #5146

Description
Prevents default focus styles from being removed, making the behaviour opt-in via <Editable>'s style prop or other CSS

Issue
Fixes: #5146

Example

Context
Since Slate.js does not provide any custom focus styles for the editable area, it should not remove the built-in focus style which is required per WCAG criterion Focus Visible

If a particular focus styling technique is preferred by library authors (e.g. highlighting the whole area with :focus-within), it should be described in the relevant documentation article (e.g. https://www.slatejs.org/examples/styling) rather than being hard-coded in the Editable component.

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: 625e4ab

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	Minor

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