Consider exposing common `Locale.NumberingSystem` as static variables

[itingliu]

Locale.LanguageCode and Locale.ScriptCode defines common language and script codes as static vars for easy access, such as

@_alwaysEmitIntoClient
public static var danish: Locale.LanguageCode { Locale.LanguageCode("da") }

We can consider exposing common Locale.NumberingSystem as well.

Discussion: https://forums.swift.org/t/fou-locale-components-language-and-language-components/54084/17