Add a nix flake, mainly for the devShell

[FollowTheProcess]

Summary

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.66%. Comparing base (e3249cf) to head (3ef2c55).

Additional details and impacted files

@@ Coverage Diff @@## main #41 +/- ##
=======================================
Coverage 86.66% 86.66% =======================================
Files 3 3 Lines 150 150 =======================================
Hits 130 130 Misses 19 19 Partials 1 1 

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