adding desktop config

modules/home-manager/defaults.nix

@@ -0,0 +1,23 @@
+{ config, lib, ... }:
+
+let
+  cfg = config.my.defaults;
+  mkCmdOption = description: lib.mkOption {
+    type = lib.types.nullOr lib.types.str;
+    default = null;
+    inherit description;
+  };
+in
+{
+  options.my.defaults = {
+    terminal = mkCmdOption "Command for the default terminal ($TERMINAL).";
+    browser  = mkCmdOption "Command for the default browser ($BROWSER).";
+    editor   = mkCmdOption "Command for the default editor ($EDITOR and $VISUAL).";
+  };
+
+  config.home.sessionVariables = lib.mkMerge [
+    (lib.mkIf (cfg.terminal != null) { TERMINAL = cfg.terminal; })
+    (lib.mkIf (cfg.browser != null)  { BROWSER = cfg.browser; })
+    (lib.mkIf (cfg.editor != null)   { EDITOR = cfg.editor; VISUAL = cfg.editor; })
+  ];
+}