act should work without mock Scheduler (#21714)

Currently, in a React 18 root, `act` only works if you mock the
Scheduler package. This was because we didn't want to add additional
checks at runtime.

But now that the `act` testing API is dev-only, we can simplify its
implementation.

Now when an update is wrapped with `act`, React will bypass Scheduler
entirely and push its tasks onto a special internal queue. Then, when
the outermost `act` scope exists, we'll flush that queue.

I also removed the "wrong act" warning, because the plan is to move
`act` to an isomorphic entry point, simlar to `startTransition`. That's
not directly related to this PR, but I didn't want to bother
re-implementing that warning only to immediately remove it.

I'll add the isomorphic API in a follow up.

Note that the internal version of `act` that we use in our own tests
still depends on mocking the Scheduler package, because it needs to work
in production. I'm planning to move that implementation to a shared
(internal) module, too.

Author: acdlite

fixtures/dom/src/__tests__/nested-act-test.js

@@ -48,7 +48,7 @@ describe('unmocked scheduler', () => {
       TestAct(() => {
         TestRenderer.create(<Effecty />);
       });
-      expect(log).toEqual(['called']);
+      expect(log).toEqual([]);
     });
     expect(log).toEqual(['called']);
   });