Standardize support for inflection across ICU APIs

[richgillam]

ICU has APIs to vend a lot of different strings that might end up being put into the middle of a sentence and needing to be inflected for the context. Examples include:

• Language, region, and script names (etc.)
• Month and and day names (and possibly other parts of a date or time)
• Formatted dates containing words (e.g., "24 de Diciembre")
• Formatted dates containing (or potentially containing) ordinal numbers ("1er de Avril")
• Dimensioned numbers ("6 feet 3 inches" or "12 pts")
• Currency names and currency values
• Person names
• Spelled-out quantities ("One hundred twenty-four")

In addition to grammatical inflection, we might also need to care about capitalization and punctuation (e.g., "Your pizza will be ready in 3 min.." if the abbreviation is handled naively).

Support for inflection in the different APIs involved varies widely and isn't expressed in the same way everywhere it is supported. We need to support the same feature set across all relevant parts of API and standardize on one way of expressing these concepts in the API.