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[Fiber] Animation priority work (#7466)

* High priority work

Adds the ability to schedule and perform high priority work. In the
noop renderer, this is exposed using a method `performHighPriWork(fn)`
where the function is executed and all updates in that scope are given
high priority.

To do this, the scheduler keeps track of a default priority level.
A new function `performWithPriority(priority, fn)` changes the default
priority before calling the function, then resets it afterwards.

* Rename overloaded priority terms

"High" and "low" priority are overloaded terms. There are priority
levels called HighPriority and LowPriority. Meanwhile, there are
functions called {perform,schedule}HighPriWork, which corresponds
to requestAnimationFrame, and {perform,schedule}LowPriWork, which
corresponds to requestIdleCallback. But in fact, work that has
HighPriority is meant to be scheduled with requestIdleCallback.
This is super confusing.

To address this, {perform,schedule}HighPriWork has been renamed
to {perform,schedule}AnimationWork, and
{perform,schedule}LowPriWork has been renamed to
{perform,schedule}DeferredWork. HighPriority and LowPriority
remain the same.

* Priority levels merge fix

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