LibWeb: Implement the rest of the Adoption Agency Algorithm

This gets us 2 points on html5test.com :^)
- Before: https://html5te.st/4cf57659bc08272e (208)
- After: https://html5te.st/fb8a9259bda1c115 (210)

Userland/Libraries/LibWeb/HTML/Parser/HTMLParser.h

@@ -120,7 +120,7 @@ private:
         RefPtr<DOM::Node> insert_before_sibling;
     };
 
-    AdjustedInsertionLocation find_appropriate_place_for_inserting_node();
+    AdjustedInsertionLocation find_appropriate_place_for_inserting_node(RefPtr<DOM::Element> override_target = nullptr);
 
     DOM::Text* find_character_insertion_node();
     void flush_character_insertions();