LibWeb: Implement getting "descendant navigables" of a document

Author: https://github.com/awesomekling
Commit: 7af09481f5
Pull-request: https://github.com/SerenityOS/serenity/pull/19535

Userland/Libraries/LibWeb/DOM/Document.h

@@ -445,6 +445,9 @@ public:
     // https://html.spec.whatwg.org/multipage/browsers.html#list-of-the-descendant-browsing-contexts
     Vector<JS::Handle<HTML::BrowsingContext>> list_of_descendant_browsing_contexts() const;
 
+    Vector<JS::Handle<HTML::Navigable>> descendant_navigables();
+    Vector<JS::Handle<HTML::Navigable>> inclusive_descendant_navigables();
+
     // https://html.spec.whatwg.org/multipage/window-object.html#discard-a-document
     void discard();