LibHTML: Add virtual Node::tag_name()

This is analogous to the DOM's Node.tagName and makes it easy to ask
"hey, what kinda thing is this Node?"

Libraries/LibHTML/DOM/Text.h

@@ -10,6 +10,8 @@ public:
 
     const String& data() const { return m_data; }
 
+    virtual String tag_name() const override { return "#text"; }
+
 private:
     String m_data;
 };