Drupal 8 add external link field class

mytheme.php

<?php

use Drupal\Core\Render\Element;

/**
 * Implements template_preprocess_field().
 *
 * @param array $variables
 */
function MYTHEME_preprocess_field(&$variables) {

  $element = $variables['element'];

  if ($element['#field_type'] === 'link') {

    $items = Element::children($element);

    foreach ($items as $key) {

      $options = $element[$key]['#options'];

      if (isset($options['external']) && $options['external'] === TRUE) {
        
        $variables['items'][$key]['content']['#options']['attributes']['class'][] = 'link-external';
      }
    };
  }
}