Add emscripten_queue_microtask

[sbc100]

This change adds a new emscripten_queue_microtask API to match the long standing web API (also available in node).

See https://developer.mozilla.org/en-US/docs/Web/API/Window/queueMicrotask

For now, I'm adding this without a polyfill so usage of the API will crash on very old targets. We could potentially polyfill using something like setTimeout, but the semantics are a little different so I'm not sure it worth doing.

Split out from #24481

[sbc100]

It turns out that this probably is not needed for #24481, but perhaps its worth adding anyway? I'm not sure.

Also, apparently this is polyfillable using Promise.resolve().then(...)

[dschuff]

I think that this makes sense given the other stuff that we have in eventloop.h
Should this also go in html5.h and its documentation?

[kripken]

lgtm with docs

[sbc100]

Abandoning for now. Hopefully not needed since Prosmise.resolve().then( is at least as good and more portable.

[dschuff]

Does it make sense to have a C API to do this? (even if it's just backed by Promise.resolve().then )?

[sbc100]

Does it make sense to have a C API to do this? (even if it's just backed by Promise.resolve().then )?

I'm not sure .. it depends how hard it is to write Promise.resolve().then using the current promise C API.. I'll look into that.