Some time zones, like "Asia/Shanghai", use a set of DST rules that end
before present day. In these cases, we should fall back to last possible
RULE entry from the TZDB. The time zone compiler published by IANA (zic)
performs the same fallback starting with version 2 of the time zone file
format.
Instead of only having dummy functions that don't work with any input,
let's at least support one time zone: 'UTC'. This matches the basic
Temporal implementation for engines without ECMA-262, for example.
This is a rather naive implementation, but serves as a first pass at
determining the GMT offset for a time zone at a particular point in
time. This implementation ignores DST (because we are not parsing any
RULE entries yet), and ignores any offset patterns of the form "Mon>4"
or "lastSun".
