Support obtaining BCP-47 ids from metazone names

[Manishearth]

On Fuchsia, the Dart SDK uses createTimeZone() with metazone names obtained from the OS (usage site).

ICU4X currently only supports this stuff with BCP-47 ids. We should have a way to go from metazone names to BCP-47 ids.

I suspect this is already part of the plan but I'm not sure if there's a specific issue filed (@nordzilla?)

cc @sffc

[sffc]

In the link you posted, it shows "America/New_York", which is an IANA time zone name, not a metazone name. Did you mean to ask about IANA-to-BCP47 mapping? That would be #2909

[Manishearth]

⇒ #2909