Undefined $mode property in bbcode_firstpass class

Here https://github.com/phpbb/phpbb/blob/3.3.x/phpBB/includes/message_parser.php#L307 and in several more places of the class bbcode_firstpass the code refers $this->mode which is defined in child parse_message class but not in the parent bbcode_firstpass class.

Also it causes PHP 8.0.0 tests to fail here https://travis-ci.org/phpbb/phpbb/jobs/630024692#L650