Add support for Scheduler#postTask

[armanbilge]

https://developer.mozilla.org/en-US/docs/Web/API/Scheduler/postTask

[armanbilge]

Needs CI fix in #126. ✅ rebased

[djspiewak]

I think this is great but can we get a bit of a warm fuzzy for whether or not this is actually a good idea? Like, is this actually faster than the other relevant polyfill branches?

[armanbilge]

whether or not this is actually a good idea

Well, putting aside measured empirical performance, this is using the Correct™ API for this sort of thing, instead of "abusing" some kind of message passing API. Indeed, the other polyfills require a supporting Map in order to pull off that trick.

scala-js-macrotask-executor/core/src/main/scala/org/scalajs/macrotaskexecutor/MacrotaskExecutor.scala

Lines 51 to 53 in 2614818

	var nextHandle = 1
	val tasksByHandle = mutable.Map[Int, () => Unit]()
	var currentlyRunningATask = false