Add: scroll while dragging

[souporserious]

First, thank you for such a wonderful site! I'm in the middle of making a drag and drop library and have been splitting up the architecture as I go along and thought it might be nice to have an example of scrolling the viewport while dragging. I have a React example here that I wouldn't mind converting to vanilla if you think it would make for a good example. Please let me know, I'd be happy to start a PR 🙂