Hierarchical taxonomy checkboxes

@stella Adding this element makes complete sense to me.

Besides tests, we should consider....

• Improve code reuse by creating a WebformTermBase form element and plugin, which would be extended by WebformTermCheckboxes and WebformTermSelect.
• Creating a WebformTermRadios element.
• Change 'test_element_term_select' test form to 'test_element_term' and add instance of 'webform_term_checkboxes' and 'webform_term_radios' elements.
• Since the list of displayed terms could get pretty tall and nice enhancement would be to make the "term tree" scrollable by settings customizable max height to the div container.

Notes.

• Once these new elements are finalized, the webform_test.module needs to be updated to generate examples of these new elements.

I think creating a WebformTermBase class is the key task and creating a WebformTermRadios element can be handled in a new ticket.

@stella I am going to do a little work on the patch.

@stella I added support for the #scroll property and set up a WebformTermReference trait. I ran into some "out of sync" exported configuration so the attached patch is updating all exported config.

Below is a screenshot of the #scroll adding scrolling to term checkboxes.

Right now, I can't see supporting the custom #states hide/show logic but below is your code moved into a custom hook, that you can use to maintain this functionality for your client.

/**
 * Implements hook_webform_element_ELEMENT_TYPE_alter().
 */
function CUSTOM_MODULE_webform_element_webform_term_checkboxes_alter(array &$element, \Drupal\Core\Form\FormStateInterface $form_state, array $context) {
  $taxonomy_storage = \Drupal::entityTypeManager()->getStorage('taxonomy_term');
  $tree = $taxonomy_storage->loadTree($element['#vocabulary']);
  $name = $element['#webform_key'];
  foreach ($tree as $item) {
    if (!empty($item->parents)) {
      // We only care about the first parent for adding classes.
      $parent = $item->parents[0];
      $element[$item->tid]['#attributes']['class'][] = 'parent-' . $parent;
      $element[$item->tid]['#states'] = [
        'visible' => [
          ":input[name=\"{$name}[{$parent}]\"]" => ['checked' => TRUE],
        ],
      ];
    }
  }
}

I am going to write some tests and commit this improvement.

In the long term, it would be really cool if we could support something like https://www.jstree.com/ for selecting hierarchical taxonomy terms.

@stella I also think it should be possible to add support to the webform module for the cshs module which includes a dedicated form element.

@stella That requirement makes sense. I remember a similar feature being included in the hierarchical select module. I think the UX would need to more refined before being included in the Webform core module. For example, any term with sub-terms should have an open/close indicator, like an arrow. Hopefully, the webform module can just integrate with one of the available hierarchical selection module.

For now, the scrolling behavior will allow for 100+ terms to be displayed.

BTW, this custom hide/show code could be moved to a recipe.