Share attribute conversion from MDXHAST to JSX with hast-to-hyperscript

[johno]

As brought up in #715, we're currently duplicating a lot of logic with hast-to-hyperscript. We should investigate how we can share the implementation, possibly by breaking out some of the logic into another file or package.

[johno]

Going to close this since it's landed in the next branch for v2.