fix: flush onDOMSelectionChange on onDOMBeforeInput

[BitPhinix]

Description
Fixes the same fundamental issue as #4669 did, only triggered in a slightly different way. If the editor isn't focused the first select causes the 2nd one that Grammarly causes to be delayed by the onDOMSelectionChangeonDOMSelectionChange throttle.

Issue
Fixes: #4801

Example
Before:

After:

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: 4315c3d

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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[BitPhinix]

Integration tests pass locally, something seems to be up with ci. Maybe just needs a re-run.