Support translation entities

@batkor thanks a lot! I'm a bit afraid of the array deep merge. Did you test this carefully?

Would it be possible to have tests for such cases to ensure it works as expected, now and in the future?

Just added kernel tests for the translated nodes that references terms to be merged.

- Drupal\Tests\term_reference_change\Kernel\ReferenceTranslatedFinderTest::findsReferencingEntities
- Drupal\Tests\term_reference_change\Kernel\ReferenceTranslatedFinderTest::migrateTranslatedReferencingEntitiesSingle
- Drupal\Tests\term_reference_change\Kernel\ReferenceTranslatedFinderTest::migrateTranslatedReferencingEntities"original has synonyms"
- Drupal\Tests\term_reference_change\Kernel\ReferenceTranslatedFinderTest::migrateTranslatedReferencingEntities"translated has synonyms"
- Drupal\Tests\term_reference_change\Kernel\ReferenceTranslatedFinderTest::migrateTranslatedReferencingEntities"both have synonyms"
- Drupal\Tests\term_reference_change\Kernel\ReferenceTranslatedFinderTest::migrateTranslatedReferencingEntities"no synonyms"

The migrateTranslatedReferencingEntities test is a little overkill and may be hard to understand because the logics to accept dataprovider, but it also helps to be sure that things are working in most of the scenarios.
The migrateTranslatedReferencingEntitiesSingle test is the same but limited to one scenario.
The findsReferencingEntities just find the referencing entities with terms to be merged.

All have passed in local, let's see here. So it could be reviewed.

As the tests in ci are failing... I'll check asap

Finally passing, waiting for review now.