feat(developer): add a BCP 47 picker for languages in Package Editor, New Project

The current language selector in the Package Editor and New Project dialogs is very rudimentary. It forces a 'holy triplet' lang-script-region style BCP 47 tag (only lang is reqd) and does not allow extensions. It does not have a search for subtags. It needs to support arbitrary subtags (probably not grandfathered tags).