Return if locale was found or not

When you call .best(locale), previously we return the default locale if nothing is found. If the user needs to know if a matching locale was found, they could set locale.Locale.default = "unknown" or something similar.

When multiple versions of Locale are in memory, it's possible for require() to cache the instance and you can end up with conflicts with the default.

This is a solution around that problem by adding an additional returned value, `defaulted` that specifies if a match was found.