Add serializer for json/xml

Problem/Motivation

Quoting Wim Leers at #2543726-247: Make $term->parent behave like any other entity reference field, to fix REST and Migrate support and de-customize its Views integration:

I followed the steps in #246 (albeit without using drush). I reproduced it.

The root cause is not Commerce, nor this patch. It's the entity_reference_revisions module's \Drupal\entity_reference_revisions\EntityReferenceRevisionsServiceProvider. That will need to be updated.
(And if that code used the service decorator pattern, it wouldn't even need to be updated. Look at the html_response.attachments_processor.big_pipe service for an example of that.)

That was wrong, see #3, but using the parent: … pattern is possible. It still achieves the same: removing the need to keep this subclass' service's arguments in sync with the parent's.

Proposed resolution

Use parent: … pattern, then you won't have to update this again in the future.

Remaining tasks

User interface changes

API changes

Data model changes