runtime: async preemption (or alternative) for js/wasm

[prattmic]

This is split from #65178, as the specific scheduler bug was fixed.

Due to its single-threaded nature, js/wasm have no sysmon thread, and thus no asynchronous preemption. Thus the pitfalls of Go prior to asynchronous preemption apply: tight loops that fail to yield may delay scheduling indefinitely.

This is a tracking issue for problems due to the lack of asynchronous preemption. As noted in #36365, due to the lack of threading, asynchronous preemption isn't possible today, but perhaps could be with wasm threads? Or we could have an alternative scheme for js/wasm (such as adding preemption points to loops).

(Plan9 is also missing async preemption, but that simply needs an implementation)