Drush 9 commands

@robpowell We should consider updating the drush 8 commands to use the drush 9 class-based approach so we only have to maintain a single set of commands.

A suggestion for this patch would be to avoid backslashes in translated text. It's easier to read if you simply enclose the text in double quotes, which would make escaping the single quotes unnecessary.

See the coding standards doc:
https://www.drupal.org/docs/develop/standards/coding-standards#quotes

    if (empty($type)) {
      $this->output()->writeln(dt('Type value must be either \'monitor\' or \'task\'. Please resubmit with one of these values.'));
      return;
    }

    $tags = $options['tags'];
    $tags = !empty($tags) ? explode(',', $tags) : [];
    $list = !empty($list) ? explode(',', $list_to_run) : [];

    $approved_type = in_array($type, ['monitor', 'task']);
    if (!$approved_type) {
      $this->output()->writeln(dt('Type value must be either \'monitor\' or \'task\'. Please resubmit with one of these values.'));
      return;
    }

I have committed the changes here with a couple of small updates. It appears a single existence check was calling a non-existent variable.