Expose isPending from startTransition by using useTransition hook

[akamfoad]

Currently RR uses React.startTransition so if you have a clientLoader that returns promises and doesn't await for anything, the navigation.state goes back to idle immediately, making it hard to show loading feedback while using with Suspense.

Userland code has to wrap the navigation calls from RR in another startTransition from useTransition in order to access the isPending state.

If React Router switch to useTransition internally, its possible to expose a boolean like isTransitioning from the useNavigation hook that can be used by userland code, avoiding adding unnecessary code.

My main use case is that, if I have a page with some static content, a dynamic table and pagination content, I want to show a spinner when initially loading the page in place of the body and the pagination, but when user changes some search parameters (seach, page, etc...) I want only the body page to show the spinner, but the rest of the page, including the pagination stay as is.

The key on the Suspense should've done the trick, but React never suspend the Suspense wrapping the body when there are other sibling Suspense boundaries that hasn't suspended (namely, the Suspense wrapping the Pagination).

Even though React could improve this, I think exposing the transition state is still a good addition.

Here is a Stackblitz example: https://stackblitz.com/edit/react-router-react-19-suspense?file=app%2Froutes%2Fno-key-and-use-transition.tsx