Add Nix flake

[adisbladis]

This is another take on #1026 that aims to minimally implement a Nix development environment.

Notable differences are:

• Uses pyproject.nix for auto-packaging of the environment

  • Not only is mach-nix unmaintained, but it's also very heavy weight.

  • Uses nixpkgs Python packages Which means better binary cache performance than mach-nix offers.

• No direnv additions I use direnv myself but expect users to be able to echo 'use flake' > .envrc if they want to use direnv.

• Uses zephyr sources from a flake input This should make maintenance simpler as there is no manual updating required.

[kilipan]

Would love for a nix flake to be added, too!