Overriding of internal transliterators

[skius]

We do not allow users to directly create transliterators (using the runtime API) that are any of:

1. Wrappers around existing components, like Any-Upper (for uppercasing), Any-NFC, ...
2. Transliterators from sources marked as visibility: internal, such as Bengali-InterIndic (nvm - see comment)

Using the "plugin"/"override"/"custom code-based" transliterator functionality where users can override the implementation of a transliterator at runtime using a trait object, can users override these impossible-to-construct transliterators?

ICU seems to support overriding both transliterators like Any-NFC and visibility: internal ones like Bengali-InterIndic.

[skius]

Blocking overrides of visibility: internal ones is not reasonable - if a user doesn't have the data for a transliterator (because they're overriding it), the constructor also won't know if it would have been an internal transliterator or not.

[sffc]

Needs design work.

[sffc]

May need CLDR work in order to define the official BCP-47 tags.