Missing typehints [#3594904]

Problem/Motivation

The source plugin is missing at least one typehint, leading to errors such as:

  PHPUnit\Framework\Exception: Fatal error: Declaration of Drupal\migrate_directory\Plugin\migrate\source\MigrateDirectory::rewind() must be compatible with Drupal\migrate\Plugin\migrate\source\SourcePluginBase::rewind(): void in /opt/drupal/web/modules/contrib/migrate_directory/src/Plugin/migrate/source/MigrateDirectory.php on line 25

Steps to reproduce

Attempt to run with PHP 8.4.

Proposed resolution

Add the typehint(s).

Remaining tasks

get MR together
get it merged
get it released

User interface changes

API changes

Data model changes

Issue fork migrate_directory-3594904

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3594904-missing-typehints changes, plain diff MR !3
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