react-reconciler: flushSync was renamed to flushSyncFromReconciler without deprecation notice

[remorses]

In react-reconciler, the flushSync export was renamed to flushSyncFromReconciler in PR #28500 (merged April 8, 2024). This is an undocumented breaking change for custom renderers.

Commit: #28500

According to that PR, custom renderers should now use:

• reconciler.updateContainerSync() - to update synchronously
• reconciler.flushSyncWork() - to flush pending sync work

However, this migration path is not documented anywhere for custom renderer authors. The change silently causes flushSync to be undefined with no warning.

0.29.x API:

const reconciler = ReactReconciler(hostConfig)
const { flushSync } = reconciler  // works

// Usage
flushSync(() => {
  reconciler.updateContainer(element, container, null, callback)
})

0.31.0+ API:

const reconciler = ReactReconciler(hostConfig)
const { flushSync } = reconciler  // undefined!

// New intended usage (undocumented):
reconciler.updateContainerSync(element, container, null, callback)
reconciler.flushSyncWork()

Could we please:

1. Document the new API for custom renderers in the react-reconciler README
2. Or add a deprecation warning when accessing the old flushSync property

This affects custom renderers like @opentui/react that were following the previous patterns.

[pranamya123]

This PR addresses the issue described here:

#35424

It documents the removal of flushSync from react-reconciler, adds guidance on the new replacement APIs (updateContainerSync and flushSyncWork), and improves developer experience for custom renderer authors.

See the PR for details: #35472