LibWeb: Change HTMLToken::m_doctype into named DoctypeData struct

This is in preparation for an upcoming storage change of HTMLToken. In
contrast to the other token types, the accessor can hand out a mutable
reference to allow users to change parts of the DoctypeData easily.

Userland/Libraries/LibWeb/HTML/Parser/HTMLToken.cpp

@@ -16,7 +16,7 @@ String HTMLToken::to_string() const
     case HTMLToken::Type::DOCTYPE:
         builder.append("DOCTYPE");
         builder.append(" { name: '");
-        builder.append(m_doctype.name);
+        builder.append(doctype_data().name);
         builder.append("' }");
         break;
     case HTMLToken::Type::StartTag: