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

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

• \Drupal\datetime_range\Plugin\Field\FieldType\DateRangeItem

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