fix macos accent menu when using arrow keys

[BitPhinix]

Description
Fixes inserting accented characters using the macOS accent menu when using arrow keys

Issue
Fixes: #5038

Example

Before:

After:

Context

When holding down 'a' to type a with an accent the first 'ArrowLeft' key down is triggered with isComposing=false and before any event indicating that it is composing. Immediately afterward the 'normal' composition start/update events are fired and all following events have isComposing properly set, and because the composition start is fired before the selection change selecting the character / after the beforeinput we keep the selection obtained by manually handling the arrow keys which doesn't match the dom state and insert there because chrome doesn't fire the insertFromComposition.

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: 8164385

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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