LibWeb: Add document.querySelector()

Author: https://github.com/linusg
Commit: 67b742bf32
Pull-request: https://github.com/SerenityOS/serenity/pull/2380

Libraries/LibWeb/DOM/Document.h

@@ -116,6 +116,7 @@ public:
     void schedule_style_update();
 
     Vector<const Element*> get_elements_by_name(const String&) const;
+    RefPtr<Element> query_selector(const StringView&);
     NonnullRefPtrVector<Element> query_selector_all(const StringView&);
 
     const String& source() const { return m_source; }