LibWeb: Implement "document-tree child navigables"

Author: https://github.com/awesomekling
Commit: 234a73e43e
Pull-request: https://github.com/SerenityOS/serenity/pull/20725

Userland/Libraries/LibWeb/DOM/Document.h

@@ -465,6 +465,7 @@ public:
     Vector<JS::Handle<HTML::Navigable>> inclusive_descendant_navigables();
     Vector<JS::Handle<HTML::Navigable>> ancestor_navigables();
     Vector<JS::Handle<HTML::Navigable>> inclusive_ancestor_navigables();
+    Vector<JS::Handle<HTML::Navigable>> document_tree_child_navigables();
 
     void destroy();