CLDR plural rules in Swift, as a first step toward relative dates without ICU

Following up on What's in the 34 MB of ICU data in lib_FoundationICU.so, where moving formatting into Swift came up as a better path than trimming ICU data. I've been working on one piece of that and would like feedback before going further. What swiftlang/swift-foundation#2340 adds the CLDR plural rules ("1 day" vs "2 days") in Swift, as package API in _FoundationInternationalizationData, behind a FOUNDATION_PLURAL_RULES_SWIFT flag. It's a draft, and I'm happy to change its shape or approach. ...