gpui: Flash menu in menubar on macOS when action is triggered (#38588)

On macOS, traditionally when a keyboard shortcut is activated, the menu
in the menu bar flashes to indicate that the action was recognised.

<img width="289" height="172" alt="image"
src="https://github.com/user-attachments/assets/a03ecd2f-f159-4f82-b4fd-227f34393703"
/>

This PR adds this functionality to GPUI, where when a keybind is pressed
that triggers an action in the menu, the menu flashes.

Release Notes:

- N/A

crates/gpui/src/platform.rs

@@ -250,6 +250,7 @@ pub(crate) trait Platform: 'static {
     fn on_app_menu_action(&self, callback: Box<dyn FnMut(&dyn Action)>);
     fn on_will_open_app_menu(&self, callback: Box<dyn FnMut()>);
     fn on_validate_app_menu_command(&self, callback: Box<dyn FnMut(&dyn Action) -> bool>);
+    fn on_action_triggered(&self, action: &dyn Action);
 
     fn compositor_name(&self) -> &'static str {
         ""