Rough outline of what needs to be done here:

• Create and upload binary distributions for GHCJS
• Modify Stack.Setup to know how to install these binary distributions. If they work the same as the normal GHC ones, then this change should be pretty minimal:
  • Add soptsUseGHCJS :: Bool to SetupOpts
  • Most spots that use the string "ghc" will likely need to instead use "ghcjs" when this bool is true.
  • When downloading, get the url from a new ghcjs section of stack-setup.yaml instead of the ghc section.
  • Probably more I can't anticipate
• Add URLs for the binary distributions to https://raw.githubusercontent.com/fpco/stackage-content/master/stack/stack-setup.yaml
  • NOTE: make sure that extending this won't cause older stack versions to fail