Provide a cart view mode for each relevant entity type

Each entity type that can be used as a line item source (such as commerce_product_variation) must be given a "cart" view mode.

We'll need to do this by implementing hook_rebuild() in the cart module.

Process:
1) Take the entity manager from \Drupal::service('entity.manager')
2) Find a list of relevant entity types:

    $entityTypes = \Drupal::service('entity.manager')->getDefinitions();
    $sourceEntityTypes = array_filter($entityTypes, function($entityType) {
      return $entityType->isSubclassOf('\Drupal\commerce\LineItemSourceInterface');
    });

3) For each entity type id, try to load the "cart" view mode, using EntityViewMode::load($entity_type . '.cart').
4) If missing, create it, using EntityViewMode::create() and then $entity->save().

If this works, you will be able to clear the caches, go to the "Manage display" tab of a product variation type, and see a Cart view mode.