Copy options object for PlainYearMonth.{add,subtract} and InterpretTemporalDateTimeFields

[cjtenny]

… to prevent user-modified objects from interfering with later operations.

Fixes #1686

[codecov]

Codecov Report

Merging #1748 (0b44710) into main (8b90758) will increase coverage by 0.17%.
The diff coverage is 100.00%.

❗ Current head 0b44710 differs from pull request most recent head e9fe915. Consider uploading reports for the commit e9fe915 to get more accurate results

@@            Coverage Diff             @@
##             main    #1748      +/-   ##
==========================================
+ Coverage   94.79%   94.96%   +0.17%     
==========================================
  Files          19       19              
  Lines       10963    10962       -1     
  Branches     1713     1740      +27     
==========================================
+ Hits        10392    10410      +18     
+ Misses        556      537      -19     
  Partials       15       15              

Flag	Coverage Δ
test262	78.78% <100.00%> (-2.40%)	⬇️
tests	90.11% <100.00%> (+0.57%)	⬆️

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Impacted Files	Coverage Δ
polyfill/lib/ecmascript.mjs	95.07% <100.00%> (+0.09%)	⬆️
polyfill/lib/plainyearmonth.mjs	96.44% <100.00%> (+0.56%)	⬆️
polyfill/lib/calendar.mjs	94.43% <0.00%> (+0.13%)	⬆️
polyfill/lib/plainmonthday.mjs	93.91% <0.00%> (+6.75%)	⬆️

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[ptomato]

Marking the normative PR as draft to avoid accidental merging

[ptomato]

This change achieved consensus in TC39. Can be merged when the wording issues are addressed and the tests are added.

[ptomato]

A minor comment on the tests.