Fix onCompositionEnd not updating isComposing

[Elvin7CF]

Description
After compositionend event completion, isComposing fails to update.

Issue
#5576

Example
After using IME for input, mouse clicking does not trigger the update of the selection, resulting in subsequent input being placed in the wrong position.

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: 0f5b5ef

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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