Fix issue when tabbing into editor in Safari

[12joan]

Description
Fixes an issue in Safari making it imposible to focus the editor using the Tab key when the editor is empty and has a placeholder.

Issue
Fixes: udecode/plate#2315

Example
Before:
https://user-images.githubusercontent.com/4272090/229171039-c9b535b2-9100-41c9-8e81-487eafd2be35.mov

After:
https://user-images.githubusercontent.com/4272090/229171068-a57c2064-4ead-469d-9414-80b0cfc228ca.mov

Context
Safari attempts to place the selection inside the placeholder element, which resulted in a null DOM selection prior to this fix, making it impossible to type even though the editor is technically focused. The simplest solution seems to be to allow Safari to place the selection inside the placeholder, which is what this fix does. (Thanks for discovering this workaround, @hb250131!)

Notice that the caret in the second video is grey and appears at the start of the placeholder, rather than in the middle of it. This is a side effect of the selection being inside the placeholder and happens only when focusing the editor using the Tab key in Safari. It seems to be harmless, but a more robust fix would ensure that the resulting selection is consistent across browsers.

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: dd4943a

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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