This adds a GHCJS build for Dhall so that:
* We can build a `try.dhall-lang.org` to try Dhall in the
browser until the PureScript implementation comes online
* We can document how to build `dhall` using GHCJS
* We can verify that new changes to `dhall` or its dependencies
don't break GHCJS support
* CI can build and cache GHCJS-related dependencies
