programs.zathura: added 'mappings' option

[GaetanLepage]

Description

added 'mappings' option for zathura.

Checklist

• [x] Change is backwards compatible.

• [x] Code formatted with ./format.

• [x] Code tested through nix-shell --pure tests -A run.all.

• [ ] 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.

  • [ ] Added myself and the module files to .github/CODEOWNERS.