feat: add a flake.nix

[teto]

Replaces shell.nix in upcoming nix.

This is a very basic one but hopefully we can improve it later on.

[acowley]

Thank you! Not only is this helpful for all nix users, but it finally pushed me over the edge to try out flakes myself.

[teto]

Great I will try to improve it as time goes by. The nice thing is the ability to run the latest haskell-language-server (hls) since hls provides a flake. I have limited experience with https://github.com/input-output-hk/haskell.nix but it seems to solve several issues from nixpkgs' haskell ecosystem, namely there is no need to painfully override everything anymore.

[acowley]

What’s kept me with nixpkgs is having Haskell packages in the binary cache. My understanding is that cache coverage of haskell.nix is much less (just the compiler itself, iinm). Maybe that’s a bad prioritization, but it’s very satisfying when spinning up a new project to be able to immediately throw in a bunch of quality of life dependencies without worrying about compile times.