datetime, tools: generate timezone abbreviations list

We use gen-zone-abbrevs.pl script to generate file describing timezones
and their fixed ids. Please see `perldoc gen-zone-abbrevs.pl` for more
detailed usage description.

Generator uses `zone-abbrevs.txt` as an input list of all known to
the date timezone abbreviations. We use this page as original source
https://www.timeanddate.com/time/zones/

Names encoding schema used by generator script supposed to be stable
enough for input set of properly sorted zone names, including:
- military zone names;
- zone abbreviations;
- human-readable IANA zone names.

It generates _future compatible_, strictly ordered, sequential
list.

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