LibWeb: Implement HTML parser "in column group" insertion mode

Author: https://github.com/Lubrsi
Commit: 6532c1e2fa
Pull-request: https://github.com/SerenityOS/serenity/pull/2555
Reviewed-by: https://github.com/awesomekling ✅

Libraries/LibWeb/Parser/HTMLDocumentParser.h

@@ -99,6 +99,7 @@ private:
     void handle_in_select_in_table(HTMLToken&);
     void handle_in_select(HTMLToken&);
     void handle_in_caption(HTMLToken&);
+    void handle_in_column_group(HTMLToken&);
 
     void stop_parsing() { m_stop_parsing = true; }