Move suspended render logic to ensureRootIsScheduled (#26328)

When the work loop is suspended, we shouldn't schedule a new render task
until the promise has resolved. When I originally implemented this, I
wasn't sure where to put this logic — `ensureRootIsScheduled` is the
more natural place for it, but that's also a really hot path, so I chose
to do it elsewhere, and left a TODO to reconsider later.

Now it's later. I'm working on a refactor to move the
`ensureRootIsScheduled` call to always happen in a microtask, so that if
there are multiple updates/pings in a single event, they get batched
into a single operation. Which means I can put the logic in that
function where it belongs.