Allow returning AsyncIterators from resolvers 🤔

[n1ru4l]

I had this idea that is actually possible to allow returning AsyncIterators from resolvers without waiting for it to land in graphql-js. The only downside would be that typings in user code could be wrong (Promise<T> | T expected instead of AsyncIterator<T>) 😅. However, that could be easily solved with patch-package.

The idea is to wrap each field resolver (as in #94). The wrapper will then check whether the returned value is a plain value, a Promise, or an AsyncIterator. In the case of plain value and Promise it will just forward it. In case of an AsyncIterator it will (1) get the first value and resolve a Promise with it and (2) register that the live query returns an AsyncIterator for a given query path and then forwards patches to the client once new data is returned from the AsyncIterator.

[n1ru4l]

https://github.com/graphql/graphql-js/blob/156c76ed77dc0d9b2dc3c988264d44763e8334aa/src/subscription/__tests__/simplePubSub.js