Wire up support for unstable_transitions flag

[brophdawg11]

This PR wires up a new unstable_transitions flag on the routers that gives you control over the behavior of React transitions in React Router.

• unstable_transitions={undefined} - Current behavior - Router state updates are wrapped in React.startTransition
  • We have found that this has some issues in React 19 when you wrap your own navigations in startTransition (i.e., startTransition(() => navigate('/'))) because internal mid-navigation state updates aren't surfaced in the current UI (i.e., useNavigation is stale during the navigation)
• unstable_transitions={true} - Opt-into additional transition-related behavior inside React Router
  • Navigations will now also be wrapped in React.startTransition (useNavigate, Link, useSubmit, Form)
  • In React 19, Framework/Data Router state updates will also be sent through React.useOptimistic so they can surface to the UI mid-navigation
• unstable_transitions={false} - Opt-out of all usage of React.startTransition/React.useOptimistic
  • Choosing this will mean that your application is likely not fully compatible with modern React features like <Suspense>, <ViewTransition>, use(), etc.
  • This might be required for your app if you are using React.useSyncExternalStore which is not compatible with transitions (1, 2)

RFC: #12855

TODO:

• Feature detect useOptimistic since it's a react 19 API
• More tests around fetchers surfacing mid navigation

[changeset-bot]

🦋 Changeset detected

Latest commit: 8eefbbe

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 11 packages

Name	Type
react-router	Patch
@react-router/architect	Patch
@react-router/cloudflare	Patch
@react-router/dev	Patch
react-router-dom	Patch
@react-router/express	Patch
@react-router/node	Patch
@react-router/serve	Patch
@react-router/fs-routes	Patch
@react-router/remix-routes-option-adapter	Patch
create-react-router	Patch

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