Invent BCP47 IDs for hardcoded transliterators

[skius]

Hardcoded internal transliterators such as Any-Hex need a BCP-47 ID during datagen for the case where other transliterators refer to them. They also need a BCP-47 ID for creation at runtime.

Implementation of hardcoded transliterators is tracked here: #3910

Action items of this issue:

• Find a good place to define this mapping and hopefully reuse it between runtime and datagen
• Inject the mapping for hardcoded transliterators into the mapping generated by TransliteratorCollection::generate_mapping