Add CI for the JS backend

[hsyl20]

Task: add CI for GHC's JS backend (starting with 9.8, it's too broken before that).

Motivation: see discussion in #285

Running the testsuite with the JS backend is tackled in #290

[hasufell]

I've been trying this, but cabal gets stuck at Preprocessing library for unix-2.8.6.0. And then nothing.

I use GHC 9.10.0.20240413 and EMSDK 3.1.57.

[hasufell]

Ah, it just took forever.

But running the test executable yields an error:

$ dist-newstyle/build/javascript-ghcjs/ghc-9.10.0.20240413/unix-2.8.6.0/build/unix-tests/unix-tests                                       Sun 15:44
All
  Common
    fileExist01: uncaught exception in Haskell thread: ReferenceError: h$splitmix_init is not defined
ReferenceError: h$splitmix_init is not defined
    at h$$60762042 (/home/hasufell/git/unix/dist-newstyle/build/javascript-ghcjs/ghc-9.10.0.20240413/unix-2.8.6.0/build/unix-tests/unix-tests:179307:1)
    at h$runThreadSlice (/home/hasufell/git/unix/dist-newstyle/build/javascript-ghcjs/ghc-9.10.0.20240413/unix-2.8.6.0/build/unix-tests/unix-tests:14576:11)
    at h$runThreadSliceCatch (/home/hasufell/git/unix/dist-newstyle/build/javascript-ghcjs/ghc-9.10.0.20240413/unix-2.8.6.0/build/unix-tests/unix-tests:14548:12)
    at h$mainLoop (/home/hasufell/git/unix/dist-newstyle/build/javascript-ghcjs/ghc-9.10.0.20240413/unix-2.8.6.0/build/unix-tests/unix-tests:14543:9)
    at /home/hasufell/git/unix/dist-newstyle/build/javascript-ghcjs/ghc-9.10.0.20240413/unix-2.8.6.0/build/unix-tests/unix-tests:15004:13
    at h$handleErrnoC (/home/hasufell/git/unix/dist-newstyle/build/javascript-ghcjs/ghc-9.10.0.20240413/unix-2.8.6.0/build/unix-tests/unix-tests:8713:9)
    at /home/hasufell/git/unix/dist-newstyle/build/javascript-ghcjs/ghc-9.10.0.20240413/unix-2.8.6.0/build/unix-tests/unix-tests:7830:17
    at FSReqCallback.oncomplete (node:fs:187:23)