datetime.module field type stubs

[mglaman]

Part of #359. See PHPStan documentation on Stub Files.

Create stub files for the following field types in a stubs/Drupal/datetime/Plugin/Field/FieldType directory:

• \Drupal\datetime\Plugin\Field\FieldType\DateTimeItem

Add test cases to https://github.com/mglaman/phpstan-drupal/blob/main/tests/src/Type/data/field-types.php.