Wire up support for unstable_transitions flag

Author: brophdawg11

.changeset/olive-planets-think.md

@@ -0,0 +1,23 @@
+---
+"react-router": patch
+---
+
+Add new `unstable_transitions` flag to routers to give users control over the usage of [`React.startTransition`](https://react.dev/reference/react/startTransition) and [`React.useOptimistic`](https://react.dev/reference/react/useOptimistic).
+
+- Framework Mode + Data Mode:
+  - `<HydratedRouter unstable_transition>`/`<RouterProvider unstable_transition>`
+  - When left unset (current default behavior), all state updates are wrapped in `React.startTransition`
+    - ⚠️ This can lead to buggy behaviors if you are wrapping your own navigations/fetchers in `React.startTransition`
+    - You should set the flag to `true` if you run into this scenario
+  - When set to `true`, all router navigations and state changes will be wrapped
+    in `React.startTransition` and router state changes will _also_ be sent through
+    `React.useOptimistic` to surface mid-navigation router state changes to the UI (i.e., `useNavigation()`)
+  - When set to `false`, the router will not leverage `React.startTransition` or
+    `React.useOptimistic` on any navigations or state changes
+- Declarative Mode
+  - `<BrowserRouter unstable_transitions>`
+  - When left unset, all router state updates are wrapped in `React.startTransition`
+  - When set to `true`, all router navigations and state updates will be wrapped
+    in `React.startTransition`
+  - When set to `false`, the router will not leverage `React.startTransition` on
+    any navigations or state changes