feat: add nix support

[chadac]

• add flake.nix and default.nix for Nix builds

tests

• nix build .# works
• nix run .# works
• nix develop provisions a dev environment with zig

[booniepepper]

Works on the first try! 😗 👌 ✨

❯ nix run
dt 1.2.4
Learn from my mistakes - someone should.
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❯ nix develop
[hiljusti@blackbox dt]$ which zig
/nix/store/6vgflg2gmqs0cn6n3h2npimfcybizwwp-zig-0.11.0/bin/zig
[hiljusti@blackbox dt]$ zig build
[hiljusti@blackbox dt]$ zig-out/bin/dt
dt 1.2.4
Keep your stick on the ice.
» quit
[hiljusti@blackbox dt]$