Use setImmediate when available over MessageChannel (#20834)

* Move direct port access into a function

* Fork based on presence of setImmediate

* Copy SchedulerDOM-test into another file

* Change the new test to use shimmed setImmediate

* Clarify comment

* Fix test to work with existing feature detection

* Add flags

* Disable OSS flag and skip tests

* Use VARIANT to reenable tests

* lol