Add support for GHCJS

This provides a (AFAICT) fully working GHCJS package set.

@peti Can I get your feedback on how to best merge this in? I'm sure there are some things to consider, like if/how we should specify that a given package supports GHCJS (or more generically, any particular compiler).
Why is lens_4_4 required? It doesn't seem to be referenced. I don't see references to lens_4_2 either. Am I missing something?