Entities: Allow FieldItemList access through magic __get/__set methods

PHPStan said "Access to an undefined property". A custom entity may
define fields in ::baseFieldDefinitions(). Other ways to define a
field are though hook implementations like hook_entity_base_field_info(),
or through config entites created from the Drupal admin UI. Thus,
it seems very hard to track all cases in PHPStan.

While ContentEntityBase::__get() will return a FieldItemListInterface,
ContentEntityBase::__set may be used to set fields directly, thus I
decided to tell PHPStan that the type is mixed. For example, the following
code can be valid:

````php
$entity = SomeCustomEntity();
$entity->some_int_field = 34;
$entity->save();
````

Author: Christian Bläul

extension.neon

@@ -35,3 +35,6 @@ services:
 	-
 		class: PHPStan\Type\ServiceDynamicReturnTypeExtension
 		tags: [phpstan.broker.dynamicMethodReturnTypeExtension]
+	-
+		class: PHPStan\Reflection\EntityFieldsViaMagic
+		tags: [phpstan.broker.propertiesClassReflectionExtension]

src/Reflection/EntityFieldsViaMagic.php

@@ -0,0 +1,53 @@
+<?php
+
+namespace PHPStan\Reflection;
+
+use Drupal\Core\Entity\ContentEntityBase;
+use PHPStan\Type\MixedType;
+
+/**
+ * Allows field access via magic methods
+ *
+ * See \Drupal\Core\Entity\ContentEntityBase::__get and ::__set.
+ */
+class EntityFieldsViaMagic implements PropertiesClassReflectionExtension {
+
+  public function hasProperty(ClassReflection $classReflection, string $propertyName): bool {
+    return $classReflection->isSubclassOf(ContentEntityBase::class) &&
+      !$classReflection->hasNativeProperty($propertyName);
+  }
+
+  public function getProperty(ClassReflection $classReflection, string $propertyName): PropertyReflection {
+    return new class implements PropertyReflection {
+
+      public function getType(): \PHPStan\Type\Type {
+        return new MixedType();
+      }
+
+      public function getDeclaringClass(): ClassReflection {
+        return $classReflection;
+      }
+
+      public function isStatic(): bool {
+        return FALSE;
+      }
+
+      public function isPrivate(): bool {
+        return FALSE;
+      }
+
+      public function isPublic(): bool {
+        return TRUE;
+      }
+
+      public function isReadable(): bool {
+        return TRUE;
+      }
+
+      public function isWritable(): bool {
+        return TRUE;
+      }
+
+    };
+  }
+}