also refactor nixos modules

modules/nixos/shells/zsh.nix

@@ -0,0 +1,20 @@
+{
+  inputs,
+  config,
+  lib,
+  pkgs,
+  ...
+}: let
+  inherit (lib) mkIf mkEnableOption;
+
+  cfg = config.sneeuwvlok.shell.zsh;
+in {
+  options.sneeuwvlok.shell.zsh = {
+    enable = mkEnableOption "enable zsh shell";
+  };
+
+  config = mkIf cfg.enable {
+    # Enable completion for sys-packages:
+    environment.pathsToLink = ["/share/zsh"];
+  };
+}