Polyfill: Change check to assertion in sameTemporalType

[catamorphism]

This function is only called when isTemporalObject(x) is true and isTemporalObject(x) === isTemporalObject(y). Therefore, change the check for isTemporalObject(x) && isTemporalObject(y) to an assertion.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.76%. Comparing base (03f9057) to head (2037fab).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3238   +/-   ##
=======================================
  Coverage   96.75%   96.76%           
=======================================
  Files          22       22           
  Lines       10398    10398           
  Branches     1859     1858    -1     
=======================================
+ Hits        10061    10062    +1     
+ Misses        289      288    -1     
  Partials       48       48           

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