Add rollup json plugin

[chrisjsewell]

Otherwise JSON files cannot be imported

Errors here:

Seems like it should be fine to add this. I can't get anything to build without it.

Fixed up the types, should be good to go.

Originally posted by @rowanc1 in executablebooks/markdown-it-dollarmath#2 (comment)

[chrisjsewell]

Thanks @rowanc1, as discussed, we should try to push back any general package changes to here, as a shared "best practice"