[dynamicIO] Fix false-positive dynamic viewport error
When using sync IO like `new Date()` in a client component that has a
parent Suspense boundary, we should not show a prerender validation
error when `dynamicIO` is enabled.
Due to how we render the head node in parallel to the segment node, and
because the `Viewport` component that's included in the head is an async
component, we incorrectly showed the following validation error, because
the prerender was aborted before the async (and cached) viewport element
resolved.
> Route "/" has a `generateViewport` that depends on Request data
> (`cookies()`, etc...) or uncached external data (`fetch(...)`, etc...)
> without explicitly allowing fully dynamic rendering. See more info
> here: https://nextjs.org/docs/messages/next-prerender-dynamic-viewport
We can fix this by deferring the rendering of the segment node until the
scheduled microtasks have been executed. This ensures that the viewport
task is not pending when the abort occurs.
