Motivation for this change

Today after updating my nix-channel (which included the changes from #25153) I updated my personal NixOS setup to configure the zsh-syntax-highlighting properly, but I mistyped one of the highlighters which caused my shell to do odd things.

At first I feared that I broke NixOS, but in the end I've seen that it was just a stupid typo. However this showed me that it might be a good idea to replace the types.list(types.str) declaration of programs.zsh.syntax-highlighting.highlighters with a types.list(types.enum([/* available highlighters */])) to ensure stricter validation.

Although the validation is restricted here this is IMHO not a BC break due to the fact that everything will continue to work if no invalid highlighters were given (but if that happened the shell would be almost unusable).

I validated the patch by using two configurations that were deployed in a test VM:

# aborts with an evaluation failure because of the invalid highlighter
{
  programs.zsh = {
    enable = true;
    syntax-highlighting = {
      enable = true;
      highlighters = [ "main" "foobar" ];
    };
  };
}

# still works
{
  programs.zsh = {
    enable = true;
    syntax-highlighting = {
      enable = true;
      highlighters = [ "main" "brackets" ];
    };
  };
}

I booted the VMs using the following command:

NIXOS_CONFIG=`pwd`/vmtest.nix nixos-rebuild -I nixpkgs=$HOME/Projects/nixpkgs/ build-vm

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.