Extracts field values from submitted form values.
Drupal\Core\Entity\EntityInterface $entity: The entity for which the widget is being submitted.
string $langcode: The language associated to $items.
array $items: The field values. This parameter is altered by reference to receive the incoming form values.
array $form: The form structure where field elements are attached to. This might be a full form structure, or a sub-element of a larger form.
array $form_state: The form state.
public function submit(EntityInterface $entity, $langcode, array &$items, array $form, array &$form_state);