Nested `startTransition` call "takes over" and makes parent `startTransition` unable to track `isPending`.

Seb says it's a bug.

Repro:

https://codesandbox.io/s/pensive-breeze-rg70wn?file=/IndexPage.js:192-288

1. Click the button
2. `isPending` in `IndexPage.js` is `true`

However, `isPending` in `App.js` is `false`.

Expected: `isPending` in `App.js` is also `true`.
subscribe: (() => void) => () => void,
      getSnapshot: () => T,
      getServerSnapshot?: () => T,
    ): T {
      currentHookNameInDev = 'useSyncExternalStore';
      warnInvalidHookAccess();
      mountHookTypesDev();
      return mountSyncExternalStore(subscribe, getSnapshot, getServerSnapshot);
    },
    useId(): string {
      currentHookNameInDev = 'useId';
      warnInvalidHookAccess();
      mountHookTypesDev();
      return mountId();
    },
  };
  if (enableCache) {
    (InvalidNestedHooksDispatcherOnMountInDEV: Dispatcher).useCacheRefresh =
      function useCacheRefresh() {
        currentHookNameInDev = 'useCacheRefresh';
        mountHookTypesDev();
        return mountRefresh();
      };
  }
  if (enableUseHook) {
    (InvalidNestedHooksDispatcherOnMountInDEV: Dispatcher).use = function <T>(
      usable: Usable<T>,
    ): T {
      warnInvalidHookAccess();
      return use(usable);