Add nix packaging support using haskell.nix

[chaserhkj]

This PR adds nix packaging for BNFC using haskell.nix, allowing it to be integrated directly with the nix ecosystem, producing more reproducible builds as well.

Note that this does not invalidate the current stack-based build system, this is just additional nix build scripts that can be used by nix systems, quite like the Dockerfile we are already providing. This does not create multiple sources of truth for building as well since haskell.nix reads stack yaml configurations and generate build instructions on the fly.

[andreasabel]

Thanks for the contribution!

Would be great if this could be tested by a GitHub workflow (CI), so that it does not bit-rot.
Do you think you can add such a workflow?
I have seen it e.g. at https://github.com/haskell/hackage-server/blob/8a8d7391b819921f67aa446c134a2bf83da1b9b4/.github/workflows/nix-flake.yml

[chaserhkj]

Workflow is added, should be working now for both os x and linux builds using nix. A few notes:

• Currently I only added building with GHC 9.8 branch in nix workflow. Let me know if you feel building all supported versions makes more sense.
• I saw that they were testing development environments in other example workflows. But our configured development environments includes Haskell language server that needs to be compiled. And that would take a ridiculously longer period of time (>20mins), so I just disabled it for now.
• For nix flake check to pass I need to do some workarounds in flake.nix to deal with hydraJobs and doctests, see comments there.

[andreasabel]

Excellent, I'll merge this now!

I cannot promise to maintain this, unfortunately, as I lack the nix skills. So further involvement would be appreciated.