Add support for GHC's JavaScript backend

[RyanGlScott]

Currently, haskell-ci relies on this GHCJS Ubuntu PPA in order to offer installation support for GHCJS. However, this PPA has not been updated since Ubuntu 18.04, and the minimum version of Ubuntu that haskell-ci now supports is Focal Fossa (20.04). As such, attempting to install GHCJS via haskell-ci will always fail (see this CI job for an example).

Perhaps haskell-ci should just remove its GHCJS support to avoid this potential footgun?

[phadej]

As you notice in the comments that there's GHC's JS backend on the horizon. Eventually I'd like to support it too, so for time being I'd just leave GHCJS stuff as it is and wait to see how GHC's JS backend shapes up.

For sure we cannot use the same "compiler identifier" for JS backend, and we'll have figure out what to do when we know better. So I don't want to cleanup GHCJS stuff for now.

EDIT: in perfect world GHC JS backend would just work when corresponding GHC works (i.e. it's more of an arch/os i.e. macos vs linux vs windows or x86 vs arm, than anything else), but I don't see that happening from the beginning.

EDIT: there is "is" macos support too, which doesn't work either, there are leftovers which don't work. I'm not too eager to cleanup them, they didn't add maintenance burden so far.

[RyanGlScott]

Sounds good to me. I'll retitle this issue to track explicitly supporting GHC's JavaScript backend.