Add Export for menu_widget_builder.dart

[koutja]

I am currently working on a project that requires access to the menu_widget_builder.dart file located in the scaffold/mobile package. The reason for this request is that I need to override the DefaultMobileMenuTheme, and having direct access to this file would greatly facilitate that process. And same pls for scaffold/desktop.
Could you please add an export statement for this file?

Thank you for considering my request. I appreciate your assistance!