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/Document.h

@@ -23,6 +23,8 @@ public:
     void add_sheet(const StyleSheet& sheet) { m_sheets.append(sheet); }
     const NonnullRefPtrVector<StyleSheet>& stylesheets() const { return m_sheets; }
 
+    virtual String tag_name() const override { return "#document"; }
+
 private:
     OwnPtr<StyleResolver> m_style_resolver;
     NonnullRefPtrVector<StyleSheet> m_sheets;

Libraries/LibHTML/DOM/Element.h

@@ -26,7 +26,7 @@ public:
     explicit Element(const String& tag_name);
     virtual ~Element() override;
 
-    const String& tag_name() const { return m_tag_name; }
+    virtual String tag_name() const override { return m_tag_name; }
 
     String attribute(const String& name) const;
     void set_attribute(const String& name, const String& value);

Libraries/LibHTML/DOM/Node.h

@@ -2,6 +2,7 @@
 
 #include <AK/Badge.h>
 #include <AK/RefPtr.h>
+#include <AK/String.h>
 #include <AK/Vector.h>
 #include <LibHTML/TreeNode.h>
 
@@ -30,6 +31,8 @@ public:
     RefPtr<LayoutNode> create_layout_node(const StyleResolver&, const StyleProperties* parent_properties) const;
     RefPtr<LayoutNode> create_layout_tree(const StyleResolver&, const StyleProperties* parent_properties) const;
 
+    virtual String tag_name() const = 0;
+
 protected:
     explicit Node(NodeType);
 

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;
 };