dillon1000/react

allow nested `act()`s from different renderers (#16039)

* allow nested `act()`s from different renderers

There are usecases where multiple renderers need to oprate inside an act() scope
- ReactDOM.render being used inside another component tree. The parent component will be rendered using ReactTestRenderer.create for a snapshot test or something.
- a ReactDOM instance interacting with a ReactTestRenderer instance (like for the new devtools)

This PR changes the way the acting sigils operate to allow for this. It keeps 2 booleans, one attached to React, one attached to the renderer. act() changes these values, and the workloop reads them to decide what warning to trigger.

I also renamed shouldWarnUnactedUpdates to warnsIfNotActing

* s/ReactIsActing/IsSomeRendererActing and s/ReactRendererIsActing/IsThisRendererActing