Doesn't work with asyncify

[gaberogan]

Would it be possible for this library to support asyncify? Currently if I want to marshal an asyncified function I can't do that e.g.

  vm.arena.expose({
    http: {
      fetchSync: vm.newAsyncifiedFunction(async () => {
        return await fetch(url).then(r => r.text())
      }),
    },
  })

[rot1024]

I will check the latest update of quickjs-emscripten, but I'm busy, so it will take a little more time. (PR is anyime welcome)