LibWeb: Add some helpers to the DOM Node class

This patch adds the following things needed by the HTML spec:

- Node::child_text_content()
- Node::is_connected()
- Node::root()

Libraries/LibWeb/DOM/Node.h

@@ -94,8 +94,13 @@ public:
     const HTMLAnchorElement* enclosing_link_element() const;
     const HTMLElement* enclosing_html_element() const;
 
+    String child_text_content() const;
+
     virtual bool is_html_element() const { return false; }
 
+    const Node* root() const;
+    bool is_connected() const;
+
     template<typename T>
     const T* first_child_of_type() const;