Motivation for this change

Provides fonts (icons) for the emacs package all-the-icons. These are needed to get the package to work properly.

The emacs package now provides a way to install the fonts, but this is a stateful best guess based on the OS. A nix font package is, in my view, the correct way to handle this is NixOS, as with e.g. powerline.

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.