GMTOffsetFromTz: Determine time offset from timezone
Description
The data are from
https://www.timeanddate.com/library/abbreviations/timezones/ and were
hand-edited to develop this code, so there may be errors. Also, note that
some of these contradict; if you examine the code, you'll see some
commented-out portions that represent solving conflicting definitions by
choosing the more common timezone abbreviation over a the less common one.