[Flight] use microtask for scheduling during prerenders (#30768)

In https://github.com/facebook/react/pull/29491 I updated the work
scheduler for Flight to use microtasks to perform work when something
pings. This is useful but it does have some downsides in terms of our
ability to do task prioritization. Additionally the initial work is not
instantiated using a microtask which is inconsistent with how pings
work.

In this change I update the scheduling logic to use microtasks
consistently for prerenders and use regular tasks for renders both for
the initial work and pings.