dillon1000/react

[scheduler] Rename priority levels (#13842)

- "Interactive" -> "user-blocking"
- "Whenever" -> "Idle"

These are the terms used by @spanicker in their main-thread scheduling
proposal: https://github.com/spanicker/main-thread-scheduling#api-sketch

That proposal also uses "microtask" instead of "immediate" and "default"
instead of "normal." Not sure about "microtask" because I don't think
most people know what that is. And our implementation isn't a proper
microtask, though you could use it to implement microtasks if you made
sure to wrap every entry point. I don't really have a preference between
"default" and "normal."

These aren't necessarily the final names. Still prefixed by `unstable_`.