Add ReactDOM.unstable_createSyncRoot (#15504)

* Add ReactDOM.unstable_createSyncRoot

- `ReactDOM.unstable_createRoot` creates a Concurrent Mode root.
- `ReactDOM.unstable_createSyncRoot` creates a Batched Mode root. It
does not support `createBatch`.
- `ReactDOM.render` creates a Legacy Mode root. It will eventually be
deprecated and possibly moved to a separate entry point, like
`react-dom/legacy`.

* Rename internal ReactRoot types