Add an option to get only parent terms

Just a brief description of what i did inside the permissions_by_terms_parents.patch

- Added only_parents item inside $form array, SettingsForm.php:a

    $form['only_parents'] = [
      '#type'          => 'checkbox',
      '#title'         => t('Show only the parent terms'),
      '#description'   => t('Check this if you doesn\'t want to show terms children but only the parents.'),
      '#default_value' => $this->config('permissions_by_term.settings')->get('only_parents') ?? false
    ];
...
     $this->configFactory
      ->getEditable('permissions_by_term.settings')
      ->set('only_parents', $form_state->getValue('only_parents'))
      ->save();

- Then, before getting all terms (from multiple vocabularies or from one) checks if the only_parents is true (default value FALSE):

  ...
  $settings = \Drupal::config('permissions_by_term.settings');
  $target_bundles = $settings->get('target_bundles');
  $only_parents   = $settings->get('only_parents');
  $terms = $target_bundles
    ? $term_storage->loadByProperties(['vid' => $target_bundles] + ($only_parents ? ['parent' => 0] : []))
    : ($only_parents ? $term_storage->loadByProperties(['parent' => 0]) : $term_storage->loadMultiple());
  ...

Hi @marcoliver,
I fully agree with the proposed changes (I thought I had applied them myself, but apparently I mixed them up when creating the patch). Indeed, thank you for taking the trouble to look and fix those unclean portions of code!

Great! thank you!