dillon1000/react

[Fizz] Gracefully handle suspending in DOM configs (#26768)

E.g. if we suspend (throw a promise) in pushStartInstance today we might
have already pushed some chunks (or even child segments potentially). We
should revert back to where we were.

This doesn't usually happen because when we suspend in a component it
doesn't write anything itself, it'll always defer to som host instance
to do the writing.

There was a todo about this already but I'm not 100% sure it's always
safe when suspending. It should be safe when suspending just regularly
because it's just a noop. We might not even want "throwing a promise" in
this mechanism to be supported longer term but for now that's how a
suspend in internals.