feat: support creating iterator async

Allows early errors before the stream is created:

```js
async function * read (fd) {
  // read file here
}

const createReadStream = asyncIteratorToStream(async function (file) {
  const fd = await fs.open(file, 'r')
  return read(fd)
})

try {
  const stream = await createReadStream('foo.txt')
  stream.pipe(process.stdout)
} catch (error) {
  console.error('failed to open the file', error)
}
```

Author: julien-f

src/index.js

@@ -68,6 +68,13 @@ function asyncIteratorToStream (iterable, options) {
     }
   }
 
+  const { then } = iterable
+  if (typeof then === 'function') {
+    return then.call(iterable, iterable =>
+      asyncIteratorToStream(iterable, options)
+    )
+  }
+
   const iterator = resolveToIterator(iterable)
   const readable = options instanceof Readable ? options : new Readable(options)
   readable._destroy = async (error, cb) => {