The following table in TR-35 includes a web of fall back rules when the
requested time zone style is unavailable:
https://unicode.org/reports/tr35/tr35-dates.html#dfst-zone
Conveniently, the subset of styles supported by ECMA-402 (and therefore
LibUnicode) all either fall back to GMT offset or to a style that is
unsupported but itself falls back to GMT offset.
This adds an API to use LibTimeZone to convert a time zone such as
"America/New_York" to a GMT offset string like "GMT-5" (short form) or
"GMT-05:00" (long form).
The generator parses metaZones.json to form a mapping of meta zones to
time zones (AKA "golden zone" in TR-35). This parser errantly assumed
this was a 1-to-1 mapping.
