dillon1000/react

In work loop, add enum of reasons for suspending

This is a pure refactor, no change to behavior.

When a component throws, the work loop can handle that in one of several
ways — unwind immediately, wait for microtasks, and so on. I'm about
to add another one, too. So I've changed the variable that tracks
whether the work loop is suspended from a boolean
(workInProgressIsSuspended) to an enum (workInProgressSuspendedReason).