Add thorough tests for time zone transitions

[ptomato]

This adds test/thorough/timezones.mjs which tests certain invariants for time zone transitions in every supported time zone. (It doesn't use snapshots, because those might have to be updated when the IANA database updates.)

For every time zone, it takes the first five transitions after 1850 (which was about when time zones started being used) and the last two transitions before 2000 (which should cover one spring-forward and one fall-back transition in zones that use DST), and for each of those seven transitions, tests

• That each disambiguation value has the expected effect
• That .with({ offset }) works to switch between the two repeated times in a fall-back transition

This covers many different sizes of UTC offset shift, from 24 hours down to 4 seconds, with the most common being -1 hour and +1 hour.

See: #556, tc39/test262#3002

[ptomato]

@justingrant Can you think of any more time zone invariants that should be tested here?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.83%. Comparing base (17d7ff1) to head (5e301d0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3295   +/-   ##
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  Coverage   97.83%   97.83%           
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  Files          22       22           
  Lines       10725    10725           
  Branches     1856     1856           
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  Hits        10493    10493           
  Misses        215      215           
  Partials       17       17           

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