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`.
HooksDispatcherOnMountWithHookTypesInDEV = {
    readContext<T>(context: ReactContext<T>): T {
      return readContext(context);
    },
    useCallback<T>(callback: T, deps: Array<mixed> | void | null): T {
      currentHookNameInDev = 'useCallback';
      updateHookTypesDev();
      return mountCallback(callback, deps);
    },
    useContext<T>(context: ReactContext<T>): T {
      currentHookNameInDev = 'useContext';
      updateHookTypesDev();
      return readContext(context);
    },
    useEffect(
      create: () => (() => void) | void,
      deps: Array<mixed> | void | null,
    ): void {
      currentHookNameInDev = 'useEffect';
      updateHookTypesDev();
      return mountEffect(create, deps);
    },
    useImperativeHandle<T>(
      ref: {current: T | null} | ((inst: T | null) => mixed) | null | void,
      create: () => T,
      deps: Array<mixed> | void | null,
    ): void {
      currentHookNameInDev = 'useImperativeHandle';
      updateHookTypesDev();
      return mountImperativeHandle(ref, create, deps);