LibWeb: Use an Optional<String> to track the last HTML start tag

Using an HTMLToken object here is unnecessary because the only
attribute we're interested in is the tag_name.

Userland/Libraries/LibWeb/HTML/Parser/HTMLTokenizer.h

@@ -164,7 +164,7 @@ private:
 
     HTMLToken m_current_token;
 
-    HTMLToken m_last_emitted_start_tag;
+    Optional<String> m_last_emitted_start_tag_name;
 
     bool m_has_emitted_eof { false };