add Transition.waitFor() method

[frederikbosch]

The method Transition.waitFor() allows to wait for another promise to finish before continuing the transition into the new route. In particularly, if you want to add a view transition to the route change, this could look like this when this PR gets accepted.

router.on('routeWillChange', async (transition) => {
  const { promise, resolve, reject } = Promise.withResolvers();
  transition.waitFor(promise);

  const viewTransition = document.startViewTransition(async () => {
    resolve();
    await transition.promise;
  });
  await viewTransition.updateCallbackDone;
});

[frederikbosch]

You can now also wait for query param changes, allowing those transitions to be part of a view transition too.

[kategengler]

Since this would add public API to Ember, it requires an RFC

[frederikbosch]

Thanks @kategengler, I was not aware. I will follow up.