Use hyphens in Locales/language subtags instead of underscores (e.g., en-US)#157

The internet default (http html etc) for language tags is hyphens.
https://en.wikipedia.org/wiki/IETF_language_tag#Syntax_of_language_tags
But Hygraph currently only allows underscores…

For example:
American English should be en-US instead of en_US.
Chinese (T) should be zh-TW instead of the current zh_TW.