WIP: offsetGet() and offsetUnset()

core/lib/Drupal/Core/Config/Schema/ArrayElement.php

core/lib/Drupal/Core/Config/Schema/Mapping.php

core/lib/Drupal/Core/Config/Schema/SchemaCheckTrait.php

core/lib/Drupal/Core/Config/Schema/Sequence.php

core/lib/Drupal/Core/Config/StorableConfigBase.php

core/lib/Drupal/Core/TypedData/DataDefinition.php

core/modules/config_translation/src/ConfigMapperManager.php

...config_translation/src/Form/ConfigTranslationFormBase.php

...odules/config_translation/src/FormElement/ListElement.php

core/modules/locale/src/LocaleConfigManager.php

...tests/Drupal/KernelTests/Core/Config/ConfigSchemaTest.php