Baked data-driven transliterators should support aliases

[sffc]

For example, coming from CLDR JSON, we have

{
  "_visibility": "external",
  "_alias": "Bengali-Arabic",
  "_aliasBcp47": "und-Arab-t-und-beng",
  "_source": "Beng",
  "_direction": "forward",
  "_target": "Arab",
  "_rulesFile": "Bengali-Arabic.txt"
}

However, the alias "Bengali-Arabic" does not appear to be retained anywhere. We generate und-Arab-t-und-beng.json which does not have any aliases inside.

ICU4C supports referencing this transliterator at runtime as "Bengali-Arabic". I guess we should do, unless we just want to explicitly discourage that behavior.