Kanshi: Better configuration options

[nurelin]

Description

Update Kanshi's nix configuration to better reflect kanshi's configuration format. Mark the previous format as deprecated.

Closes https://github.com/nix-community/home-manager/issues/5374

Checklist

• [x] Change is backwards compatible.

• [x] Code formatted with ./format.

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

• Ran kanshi tests

• [x] Test cases updated/added. See example.

• [x] Commit messages are formatted like

  {component}: {description}
  
  {long description}

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

  Maintainer CC

[rycee]

Thanks! Merged to master now 🙂