stylix/darwin: add module

[auscyber]

adds a module for wallpaper and colours for darwin

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• [ ] I certify that I have the right to submit this contribution under the MIT license
• [ ] Commit messages adhere to Stylix commit conventions
• [ ] Theming changes adhere to the Stylix style guide
• [ ] Changes have been tested locally
• [ ] Changes have been tested in testbeds
• [ ] Each commit in this PR is suitable for backport to the current stable branch

[auscyber]

So, i've never made a module for stylix, and i'm getting some errors to do with callPackage not existing, and unsure if my shellScript outputs correctly but i'm unsure how i even test this?

[auscyber]

the entire thing? or just the desktoppr?