Initial dedicated face pile element

Rather than overload Flex with yet another special case, add a dedicated
element to handle this overlap instead

Co-Authored-By: Max Brunsfeld <max@zed.dev>

crates/collab_ui/src/collab_ui.rs

@@ -4,6 +4,7 @@ mod contact_finder;
 mod contact_list;
 mod contact_notification;
 mod contacts_popover;
+mod face_pile;
 mod incoming_call_notification;
 mod notifications;
 mod project_shared_notification;