polyfill: Remove ZonedDateTime from Formattable type

[fabon-f]

Temporal.ZonedDateTime can't be formatted in Intl.DateTimeFormat methods, thus the type definition is wrong.

cf. fullcalendar/temporal-polyfill#78 fullcalendar/temporal-polyfill#81

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.91%. Comparing base (a91494f) to head (d90c1c7).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3260   +/-   ##
=======================================
  Coverage   97.91%   97.91%           
=======================================
  Files          22       22           
  Lines       10367    10367           
  Branches     1805     1805           
=======================================
  Hits        10151    10151           
  Misses        197      197           
  Partials       19       19           

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[ptomato]

Thanks!