Method on_comment_part

test/basic_parser.cpp:1551–1551

            bool on_bool( bool, system::error_code& ) { return true; }
            bool on_null( system::error_code& ) { return true; }
            bool on_comment_part( string_view, system::error_code& ) { return true; }
            bool on_comment( string_view, system::error_code& ) { return true; }
        };