[#93]: Add Nix flake file

[roberth]

Including

• flake-parts for easy extension
• haskell-flake for more declarative configuration
• .envrc to load the dev environment automatically

Issue reference:

• Closes #93
• Alternative to #94

Submission Checklist:

• [ ] Have you followed the guidelines in our Contributing document (for example, is your tree a clean merge)?
• [ ] Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• [ ] Does your submission build?
• [ ] Does your submission pass tests?
• [ ] Have you run lints on your code locally prior to submission?
• [ ] Have you updated all of the cabal/nix infrastructure?
• [ ] Is this a breaking change? Have you discussed this?