Expose a hook/callback for obsolete manifest reload

[ZipBrandon]

Problem

When React Router detects that the route manifest is obsolete, it triggers a full page reload via window.location.reload(). Applications that register beforeunload event listeners (e.g., to protect against accidental navigation with unsaved changes) have no opportunity to programmatically unsubscribe those listeners before the reload is initiated.
This creates a degraded user experience:

React Router detects an obsolete manifest and attempts to reload.
The browser fires beforeunload, which the app is listening to, presenting a "Reload Site?" confirmation dialog.
If the user clicks Cancel, the reload is aborted — but the manifest is still obsolete.
Subsequent client-side navigations now result in 404s for routes that do still exist, because the stale manifest can no longer resolve them.
The app is effectively broken until the user manually refreshes or accepts the reload.

There is currently no way to distinguish a React Router manifest reload from any other unload event, and no hook to run cleanup logic before the reload is triggered.

Improvements

Adding a hook would allow applications to:

• Unsubscribe beforeunload handlers so the reload proceeds without a confirmation dialog
• Persist any unsaved state before the reload
• Run arbitrary cleanup logic