Phase 3: language packs that prefer or restrict their language #753

Part of #747. Needs phase 2 and the language tags from the classification sub-issues.

The semantics (decisions/vocabulary-model): the default loads every language. A pack prefers its language: it settles clashes where its language has a vocabulary reading. A strict pack also drops other languages' name-level readings. Packs are sets of languages, so combining them is order-independent. Vocabulary in a non-Latin script is always on, because it selects itself by script. A pack can outrank a reading derived by a shape rule (Ma. Cristina, where Ma. is María).

Open: how today's locales packs move onto this; region and script subtags beyond zh-Hans/zh-Hant; whether third parties can ship packs.

Pack-specific issues are linked here as sub-issues.