The module overrides custom entity reference field item list classes

Problem/Motivation

The module doesn't only override field item list classes extending Drupal\Core\Field\EntityReferenceFieldItemList, but also field item classes extending Drupal\Core\Field\EntityReferenceFieldItemList. This causes certain logic to not be executed. Some examples:

• Drupal\token\MenuLinkFieldItemList
• Drupal\file\Plugin\Field\FieldType\FileFieldItemList

Proposed resolution

Only override the field item list if the class is Drupal\Core\Field\EntityReferenceFieldItemList, not if it's a class extending it. We could move any overridden methods to a trait and add subclasses for the above mentioned field item list classes. We could log a notice if we encounter a subclass that isn't covered by the module.