dillon1000/react

Replace global `jest` heuristic with `IS_REACT_ACT_ENVIRONMENT` (#22562)

* Remove `jest` global check in concurrent roots

In concurrent mode, instead of checking `jest`, we check the new
`IS_REACT_ACT_ENVIRONMENT` global. The default behavior is `false`.

Legacy mode behavior is unchanged.

React's own internal test suite use a custom version of `act` that works
by mocking the Scheduler — rather than the "real" act used publicly. So
we don't enable the flag in our repo.

* Warn if `act` is called in wrong environment

Adds a warning if `act` is called but `IS_REACT_ACT_ENVIRONMENT` is not
enabled. The goal is to prompt users to correctly configure their
testing environment, so that if they forget to use `act` in a different
test, we can detect and warn about.

It's expected that the environment flag will be configured by the
testing framework. For example, a Jest plugin. We will link to the
relevant documentation page, once it exists.

The warning only fires in concurrent mode. Legacy roots will keep the
existing behavior.