user.module field type stubs

[mglaman]

Part of https://github.com/mglaman/phpstan-drupal/issues/359. See PHPStan documentation on Stub Files.

Create stub files for the following field types in a stubs/Drupal/user directory:

• \Drupal\user\StatusItem
• \Drupal\user\TimeZoneItem
• \Drupal\user\UserNameItem

These do not need @phpdoc changes since they do not add additional properties beyond their base field type classes.

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