`act` checks the environment to determine whether to fire a warning.
We're changing how this check works in React 18. As a first step, this
refactors the logic into a single function. No behavior changes yet.

If `IS_REACT_ACT_ENVIRONMENT` is set to `false`, we will suppress
any `act` warnings. Otherwise, the behavior of `act` is the same as in
React 17: if `jest` is defined, it warns.

In concurrent mode, the plan is to remove the `jest` check and only warn
if `IS_REACT_ACT_ENVIRONMENT` is true. I have not implemented that
part yet.