feat: extract some config into modules

modules/nixos/zsh.nix

@@ -0,0 +1,19 @@
+{
+  lib,
+  config,
+  ...
+}:
+{
+  options = {
+    zsh = {
+      enable = lib.mkEnableOption "zsh with ZDOTDIR in ~/.config/zsh";
+    };
+  };
+
+  config = lib.mkIf config.zsh.enable {
+    programs.zsh.enable = true;
+    environment.etc."zshenv".text = ''
+      export ZDOTDIR=$HOME/.config/zsh
+    '';
+  };
+}