added(fish): added fish nixos module

nix/modules/fish/default.nix

@@ -1,4 +1,4 @@
 _: {
   home = ./home.nix;
-  # nixos: ./nixos.nix;
+  nixos = ./nixos.nix;
 }

nix/modules/fish/nixos.nix

@@ -0,0 +1,20 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+let
+  cfg = config.modules.fish;
+in
+{
+  options.modules.fish = {
+    enable = lib.mkEnableOption "Fish";
+  };
+  config = lib.mkIf cfg.enable {
+    users.defaultUserShell = pkgs.fish;
+    programs.fish = {
+      enable = true;
+    };
+  };
+}