Support typed_relation fields

Thank you very much @jonathan_hunt. Could you please add tests, so we can be sure this works as expected for now and in the future?

I'm revisiting this as it's desired by the Islandora community.

Is it possible to add tests for a field type that's defined by another module? I don't think further tests are possible unless we mock up a shim for the 'typed_relation' field type. That seems like overkill...

Alternately, instead of testing $field_definition->getType(), we could test is_a($fieldDefinition->getClass(), 'Drupal\Core\Field\EntityReferenceFieldItemList', true). That would work both for entity reference fields and any field type that extends entity reference, including typed relation. This might prevent the need for any additional tests.

I've updated the code to check if the field class is an instance of EntityReferenceFieldItemList rather than adding a condition for another module. I agree with Rosie's comment that this does not require tests now as the current tests already check for this.

I'd rather have an Event that could add additional context/conditions to make this more robust but since that would be a considerable change, this seems to be a decent solution.