For error messages in the Windows embedding, use stderr rather than
Windows debug logging, so that it will go to the console where, e.g.,
'flutter run' will display it.
Adds a task runner, and exposes API to allow application-level runloops to know when they need to next call the API to process engine events. Internally, sends null events to wake up the app runloop when new events are scheduled to ensure the wait time is updated accordingly.
Fixes#36420
