feat: add inir-mascot companion package

[greedoftheendless]

Adds a companion Nix package that fetches and extracts the mascot art pack (354 poses/animations) from GitHub releases to share/quickshell/inir/assets/images/mascot/, so users can merge it with the main iNiR package via symlinkJoin.

Usage:

# In flake inputs
iNiR.url = "github:snowarch/iNiR/prerelease";

# In home-manager module
pkgs.symlinkJoin {
  name = "inir-with-mascot";
  paths = [ inputs.iNiR.packages.${pkgs.system}.default inputs.iNiR.packages.${pkgs.system}.inir-mascot ];
}

[greedoftheendless]

Would like any person who uses nix, to test it out. So far it works on my machine, but don't have any VM's or other machines to check it out.