FIX(container): add menu containers public re-export

Author: liketechnik

egui/src/containers.rs

@@ -9,5 +9,5 @@ pub(crate) mod window;
 
 pub use {
     area::Area, collapsing_header::CollapsingHeader, frame::Frame, popup::*, resize::Resize,
-    scroll_area::ScrollArea, window::Window,
+    scroll_area::ScrollArea, window::Window, menu::*,
 };