Issue #3442304 by thejimbirch: Use strict types in all the new code

.../lib/Drupal/Core/Config/Action/Attribute/ActionMethod.php

core/lib/Drupal/Core/Config/Action/ConfigActionException.php

core/lib/Drupal/Core/Config/Action/ConfigActionManager.php

...Drupal/Core/Config/Action/ConfigActionPluginInterface.php

...l/Core/Config/Action/DuplicateConfigActionIdException.php

core/lib/Drupal/Core/Config/Action/EntityMethodException.php

core/lib/Drupal/Core/Config/Action/Exists.php

...ction/Plugin/ConfigAction/Deriver/EntityCreateDeriver.php

...ction/Plugin/ConfigAction/Deriver/EntityMethodDeriver.php

...l/Core/Config/Action/Plugin/ConfigAction/EntityCreate.php

...l/Core/Config/Action/Plugin/ConfigAction/EntityMethod.php

.../Config/Action/Plugin/ConfigAction/SimpleConfigUpdate.php

...upal/Core/Config/Checkpoint/CheckpointExistsException.php

...Drupal/Core/Config/Checkpoint/CheckpointListInterface.php

...pal/Core/Config/Checkpoint/CheckpointStorageInterface.php

.../Drupal/Core/Config/Checkpoint/NoCheckpointsException.php

...pal/Core/Config/Checkpoint/UnknownCheckpointException.php

core/lib/Drupal/Core/DefaultContent/Existing.php

core/lib/Drupal/Core/DefaultContent/ImportException.php

core/lib/Drupal/Core/Recipe/ConfigConfigurator.php