stack 2.1 support

[pbogdan]

I'm aware this project is not maintained anymore nor is my branch in a merge-worthy state I feel but opening a PR in case anyone else finds this work useful.

I never really worked with stack beyond simple CLI usage so that's probably very much reflected in the code. And the approach taken here is very much brute-force rather than principled and that shows too :-).

This is not tested very extensively at all.

[nh2]

We're working on Stack 2 compatibility here: https://github.com/nh2/stack2nix/pull/1