stylix: adding option for other wallpapers and improvements (DRAFT)

[no-mood]

I have adapted the function to generate a wallpaper with the Nix(OS) logo, taken from misterio77/nix-colors.

The code still needs some fixes, but the general idea should be there.

I thought of adding an additional file (nix-wallpaper.nix), which will be imported instead of the previous pixel.nix. This file will contain all the function calls that generate wallpapers based on a scheme, such as pixel-wallpaper and flake-wallpaper.

As I said, it still needs some fixes, and I haven’t changed the content of pixel-wallpaper.nix because it's not clear to me how to adapt it; in my humble opinion, we could follow the structure of flake-wallpaper.