Package tear as nix flake for distribution

[expelledboy]

This is my attempt at getting this packaged via a flake, I am struggling...

If I can get make working in the dev shell, then the packaging will be trivial. But I am not that familiar enough with how xcodebuild works, if I am being honest.

I also made a few improvements along the way:

• automatic environment using direnv
• reproducible dev env using flake.nix devShell
• refactored unit tests to use bats
• refactored Makefile to include only build targets (exception clean)
• refactored tasks into Justfile
• added xcpretty for cleaner results from xcodebuild

Anyway, I am going out for drinks! :)