Polyfill: Speed up largestUnit: month date difference for calendars without leap months

[gibson042]

Fixes #3153

Note that checking against test262 will require locally adopting js-temporal/temporal-test262-runner#20, manually creating the missing directory, or a similar action.

[codecov]

Codecov Report

❌ Patch coverage is 20.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.82%. Comparing base (6274e5d) to head (6798571).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
polyfill/lib/calendar.mjs	20.00%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3154      +/-   ##
==========================================
- Coverage   96.86%   96.82%   -0.04%     
==========================================
  Files          22       22              
  Lines       10080    10084       +4     
  Branches     1824     1825       +1     
==========================================
  Hits         9764     9764              
- Misses        267      270       +3     
- Partials       49       50       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[ptomato]

Very nice. I had started looking for speedups at a lower level but this sidesteps the whole problem (for calendars with 12-month years, at least.)