SyncEngine move/renames

[schmidt-sebastian]

• Splits up sync_engine and sync_engine_impl.ts
• Prefixes all sync engine methods with syncEngine.

In two commits for easier review.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 6a50397

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[google-oss-bot]

Size Analysis Report

Affected Products

No changes between base commit (dface90) and head commit (c48ac1a).

Test Logs

• Base (dface90f): https://github.com/firebase/firebase-js-sdk/actions/runs/414416828
• Head (c48ac1af): https://github.com/firebase/firebase-js-sdk/actions/runs/415839732