GHCJS support

[patrickmn]

[aherrmann]

Dropped to P4. It's unclear if GHCJS is still needed given that we now have Asterius support.

[avdv]

Things changed a bit here: Asterius is now deprecated in favor of the native GHC WebAssembly backend (see https://www.tweag.io/blog/2022-11-22-wasm-backend-merged-in-ghc).

And also there is a GHC Javascript backend available (see https://engineering.iog.io/2022-12-13-ghc-js-backend-merged/).

Both are still experimental, but as they stabilize it would make sense to support them.