feat: set fish as user shell

modules/cli/shell/defaultShell.nix

@@ -0,0 +1,34 @@
+{
+  lib,
+  config,
+  username,
+  ...
+}:
+let
+  cfg = config.my.cli.shell;
+in
+{
+  options.my.cli.shell.default = lib.mkOption {
+    type = lib.types.enum [
+      "fish"
+      "zsh"
+    ];
+    default = "fish";
+  };
+
+  config = lib.mkMerge [
+    (lib.mkIf (cfg.default == "fish") {
+      my.cli.shell.fish.enable = true;
+      users.users.${username} = {
+        shell = config.my.hm.programs.fish.package;
+        # do not need `programs.fish.enable = true` since fish is managed by home-manager
+        ignoreShellProgramCheck = true;
+      };
+    })
+
+    (lib.mkIf (cfg.default == "zsh") {
+      my.cli.shell.zsh.enable = true;
+      users.users.${username}.shell = config.my.hm.programs.zsh.package;
+    })
+  ];
+}