[Intl] Extra timezone utilities

[ro0NL]

follow up of #11447

added Timezones::getCountryCode/forCountryCode in symfony/symfony#31262

added Timezones::getRaw/GmtOffset in symfony/symfony#31295 and symfony/symfony#31318

[rwitchell]

Hi @ro0NL - if I've got the value Asia/Kolkata, how do I use this with the symfony Timezones class (which only has Asia/Calcutta)? (it's not clear in the docs)