[Flight] Start initial work immediately (#30961)
In a past update we made render and prerender have different work scheduling behavior because these methods are meant to be used in differeent environments with different performance tradeoffs in mind. For instance to prioritize streaming we want to allow as much IO to complete before triggering a round of work because we want to flush as few intermediate UI states. With Prerendering there will never be any intermediate UI states so we can more aggressively render tasks as they complete. One thing we've found is that even during render we should ideally kick off work immediately. This update normalizes the intitial work for render and prerender to start in a microtask. Choosing microtask over sync is somewhat arbitrary but there really isn't a reason to make them different between render/prerender so for now we'll unify them and keep it as a microtask for now. This change also updates pinging behavior. If the request is still in the initial task that spawned it then pings will schedule on the microtask queue. This allows immediately available async APIs to resolve right away. The concern with doing this for normal pings is that it might crowd out IO events but since this is the initial task there would be IO to already be scheduled.