Enable purs syntax highlighting

[milesfrain]

This implements option 1 from purescript-halogen/purescript-halogen#664 (comment)

This strategy is also used in the PS book.

I notice the repo structure of this book is a bit different than the others (which don't track any autogenerated files outside of the gh-pages branch). It would probably be easier for folks to contribute to these mdbooks in the PS ecosystem if they're all architeted similarly (not claiming we found the best solution yet). Also not sure what the ferris files are for.

[ajnsit]

I discovered mdbook in the rust book repo (https://github.com/rust-lang/book) and based the structure off that. It's probably a good idea to bring it in line with the PS book.

I'll look more into how the PS highlighting code works before I merge it.