Bugfix: prop change not updating vue instance for async component

[kat3su]

For async component, props definition weren't available when setting observedAttributes causing attributeChangedCallback not firing at all.

Changing to use MutationObserver to watch for any attribute changes.

I'm not able to run npm install on this package somehow so not able to build this. Tested it in my own repo and it works so if someone can help to build this, it would be very appreciated.

Also, we should update doc to say we can use Async Component