dillon1000/react

[Scheduler] Check for continuous input events (#22107)

* [Scheduler] Track start of current chunk

Currently in `shouldYield`, we compare the current time to a deadline
that is pre-computed at the beginning of the current chunk.

However, since we use different deadlines depending on whether an input
event is pending, it makes more sense to track the start of the current
chunk and check how much time has elapsed since then.

Doesn't change any behavior, just refactors the logic.

* [Scheduler] Check for continuous input events

`isInputPending` supports a `includeContinuous` option. When set to
`true`, the method will check for pending continuous inputs, like
`mousemove`, in addition to discrete ones, like `click`.

We will only check for pending continuous inputs if we've blocked the
main thread for a non-neglible amount of time. If we've only blocked
the main thread for, say, a few frames, then we'll only check for
discrete inputs.

I wrote a test for this but didn't include it because we haven't yet set
up the `gate` flag infra to work with Scheduler feature flags. For now,
I ran the test locally.

* Review nits