LibWeb: Implement getting the traversable navigable of a navigable

Author: https://github.com/awesomekling
Commit: 05dc1da321
Pull-request: https://github.com/SerenityOS/serenity/pull/18471

Userland/Libraries/LibWeb/HTML/Navigable.h

@@ -40,6 +40,8 @@ public:
     JS::GCPtr<NavigableContainer> container() const;
     void set_container(JS::GCPtr<NavigableContainer>);
 
+    JS::GCPtr<TraversableNavigable> traversable_navigable();
+
 protected:
     Navigable();