LibHTML: Implement some very simple selector matching.

We walk the entire DOM and check all selectors against all elements. Only
id, class and tag name are checked right now. There's no ancestor stack
or compound selectors. All in good time :^)

LibHTML/DOM/Element.h

@@ -40,6 +40,8 @@ public:
             callback(attribute.name(), attribute.value());
     }
 
+    bool has_class(const StringView&) const;
+
     virtual RefPtr<LayoutNode> create_layout_node() override;
 
 private: