add an async to JS

_From @miketamis on November 24, 2018 0:24_

There needs to be a way to do async javascript in morph blocks (especially since the whole build and run time is all async) the idea at the moment is to just have it as a param ie
```morph
<js async>
const myVar = await myPromise();
</js>

```

_Copied from original issue: miketamis/morph-extension#13_