linux-wallpaperengine: add module

[ckgxrg-salt]

Description

Adds the linux-wallpaperengine systemd service.
The original project is here: https://github.com/Almamu/linux-wallpaperengine.
It simply converts options into command-line arguments and provides a systemd service to run linux-wallpaperengine in the background.

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

[ckgxrg-salt]

Oh so I have to use hm.maintainers instead of maintainers