[#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 Nix flake file #93
• Alternative to Add flake file #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?