Upstream to `nixpkgs`

[srid]

As a final step of #140, let's upstream to nixpkgs:

Preliminary

• [x] Finalize LICENSE for omnix and crates @srid
• [x] Release omnix crate to crates.io @srid

Upstream

• [ ] Upstream pkgs.omnix to nixpkgs
  • [ ] Check: it installs the om binary (not omnix-cli).
  • [ ] Check: auto completion works on zsh (macOS) and bash (Linux)
  • [ ] Check: it builds on Intel mac
    • Even if upstream doesn't test x86_64-darwin in CI, we should manually make sure that our PR builds.

[srid]

All crates have now been published:

https://crates.io/crates/omnix-cli