dillon1000/react

Set up test infra for dynamic Scheduler flags (#22139)

I copied the set up we use for React.

In the www-variant test job, the Scheduler `__VARIANT__` flags will be
`true`. When writing a test, we can read the value of the flag with the
`gate` pragma and method.

Note: Since these packages are currently released in lockstep, maybe we
should remove SchedulerFeatureFlags and use ReactFeatureFlags for both.