[Fizz] Move formatContext tracking back to the task (#27325)

In https://github.com/facebook/react/pull/21113 I moved this over to the
segment from the task. This partially reverts this two use two fields
instead. I was just trying to micro-optimize by reusing a single field.

This is really conceptually two different values. Task is keeping track
of the working state of the currently executing context.

The segment just needs to keep track of which parent context it was
created in so that it can be wrapped correctly when a segment is
written. We just happened to rely on the working state returning to the
top before completing.

The main motivation is that there is no `segment` for replaying.