Intl.js: Fix LanguageTag REs; implement some logic which depends on them. (Resolves #2981 #3652 #3653 #3654 #3648)
Intl.js: Fix LanguageTag REs; implement some logic which depends on them. (Resolves #2981 #3652 #3653 #3654 #3648)

Resolves #2981: Intl.js: Add RegExp to high-level test for valid language tags (according to grammar in RFC 5646 Section 2.1)
Resolves #3652: Intl.js fallback: platform.getExtensions
Resolves #3653: Intl.js fallback: platform.resolveLocaleLookup
Resolves #3654: Intl.js fallback: platform.resolveLocaleBestFit
Resolves #3648: Intl-ICU: implement Intl.*.supportedLocalesOf

Microsoft/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore