LibWeb: Implement getting the top-level traversable of a navigable

Author: https://github.com/awesomekling
Commit: 54e01824c7
Pull-request: https://github.com/SerenityOS/serenity/pull/18471

Userland/Libraries/LibWeb/HTML/Navigable.h

@@ -41,6 +41,7 @@ public:
     void set_container(JS::GCPtr<NavigableContainer>);
 
     JS::GCPtr<TraversableNavigable> traversable_navigable();
+    JS::GCPtr<TraversableNavigable> top_level_traversable();
 
 protected:
     Navigable();