nix-init: add module

[uncenter]

Description

Adds nix-init, a CLI for generating Nix packages from URLs with hash prefetching, dependency inference, license detection, and more.

This PR writes the configured settings to nix-init's XDG configuration file.

Checklist

• Change is backwards compatible.

• Code formatted with ./format.

• Code tested through nix-shell --pure tests -A run.all or nix develop --ignore-environment .#all using Flakes.

• Test cases updated/added. See example.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • Added myself as module maintainer. See example.

Maintainer CC