Add bindings to DataTransfer

[spocke]

We should have bindings for DataTransfer.

Relevant spec: https://html.spec.whatwg.org/multipage/dnd.html#the-datatransfer-interface

[TheSpyder]

This was PR'd to the bs-webapi repo, I forgot to port it over
reasonml-community/bs-webapi-incubator#208 and reasonml-community/bs-webapi-incubator#209

[spocke]

Looking at that PR it was less extensive. I think I covered the full spec.