Add public API to select plural of number range

We have pluralRanges.txt in ICU, and we use it internally in NumberRangeFormatter, but it would be good to expose a public API to get this information, like PluralRules::select(FormattedNumberRange), similar to PluralRules::select(FormattedNumber).

Google issue: https://yaqs.corp.google.com/eng/q/5935601535287296