Make EntityReferenceItem generic

- Allows to specify the type of entity referenced
- EntityReferenceFieldItemListInterface always contains EntityReferenceItems and so becomes generic in EntityInterface instead of FieldItemInterface

Author: jacktonkin

stubs/Drupal/Core/Field/EntityReferenceFieldItemListInterface.stub

@@ -0,0 +1,16 @@
+<?php
+
+namespace Drupal\Core\Field;
+
+use Drupal\Core\Entity\EntityInterface;
+use Drupal\Core\Field\Plugin\Field\FieldType\EntityReferenceItem;
+
+/**
+ * @template T of EntityInterface
+ * @extends FieldItemListInterface<EntityReferenceItem<T>>
+ * @property int|string|null $target_id
+ * @property ?T $entity
+ */
+interface EntityReferenceFieldItemListInterface extends FieldItemListInterface {
+
+}

stubs/Drupal/Core/Field/Plugin/Field/FieldType/EntityReferenceItem.stub

@@ -7,8 +7,9 @@ use Drupal\Core\Field\PreconfiguredFieldUiOptionsInterface;
 use Drupal\Core\TypedData\OptionsProviderInterface;
 
 /**
+ * @template T of \Drupal\Core\Entity\EntityInterface
+ * @property ?T $entity
  * @property string|integer $target_id
- * @property \Drupal\Core\Entity\EntityInterface|null $entity
  */
 class EntityReferenceItem extends FieldItemBase implements OptionsProviderInterface, PreconfiguredFieldUiOptionsInterface {
 }