EMTERPRETIFY_ASYNC: use async JS functions

[curiousdannii]

Posting this so that I don't forget that I want it.

Once library files can use beyond ES5 (probably through Babel, #6000), it would be great if async functions could be actual async JS functions, as it's simpler and would greatly reduce the boilerplate code.

What I'm imagining is that instead of the Emterpreter FFI system pausing when EmterpreterAsync.handle is called, it would check if an FFI returns a promise, and if so pause and await it. Easy to describe in simplistic terms, I'm sure it will take a lot of work :P

[stale]

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