Lateral inheritance fallback

CLDR has a concept of "Lateral Inheritance" where a value will fallback to another value before falling back to ancestor locales.

Example

ia.units.unitLength.short.length-centimeter defines only a single other key, despite ia have a plural rule that requires a one value.

When resolving the value of ia.units.unitLength.short.length-centimeter.one, it should fall back to ia.units.unitLength.short.length-centimeter.other first.

This can get very complicated if there are multiple levels of lateral inheritance.

Potential solution?

Related to #67, ruby-cldr needs to decide how much of this to handle at the thor cldr:export layer vs. exposing to clients so they can make their own decisions.

Perhaps for now, ruby-cldr should resolve values for each of the required pluralization keys for a locale (e.g., copy other for the missing plural keys), while we wait to figure out what to do about the other dimensions (e.g., "gender", "case")

Start with the CLDR Lateral Inheritance specification linked in the issue and compare it with related issue #67. Investigate the thor cldr:export entry point and decide which inheritance dimensions should be resolved during export versus exposed to clients. Done requires an agreed scope and behavior for missing pluralization keys and other dimensions such as gender and case.