add programs.zsh module

modules/programs/zsh.nix

@@ -0,0 +1,37 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+
+  cfg = config.programs.zsh;
+
+in
+
+{
+  options = {
+
+    programs.zsh.enable = mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+        Whether to configure zsh as an interactive shell.
+      '';
+    };
+
+    programs.zsh.shell = mkOption {
+      type = types.path;
+      default = "${pkgs.zsh}/bin/zsh";
+      description = ''
+        Zsh shell to use.
+      '';
+    };
+
+  };
+
+  config = mkIf cfg.enable {
+
+    environment.variables.SHELL = "${cfg.shell}";
+
+  };
+}