View Value function in email field formatter

viewValue function field formatter.php

  protected function viewValue(FieldItemInterface $item) {
    // The text value has no text format assigned to it, so the user input
    // should equal the output, including newlines.
    $url = Url::fromUri('mailto:' . $item->value);
    return Link::fromTextAndUrl(t('Send Email'), $url)->toString();
  }