Add CI pipelines for WebAssembly and Javascript targets

[supersven]

WebAssembly and Javascript targets seem to be pretty active these days. And, they seem to need some special "hacks".

IMHO it would be good to ensure ghc.nix reliably works for these targets, too, by adding GitHub CI pipelines. (Or, by adding them to existing ones. With "pipelines" I don't necessarily refer to the GitHub actions term, but to the general idea of a CI "thing".)