RefISOYear and RefISODay internal slots

[ptomato]

Temporal.YearMonth gets a RefISODay internal slot and Temporal.MonthDay
gets a RefISOYear internal slot, as discussed in #391. This will be
required for calendar support.

[codecov]

Codecov Report

Merging #627 into main will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #627   +/-   ##
=======================================
  Coverage   97.82%   97.83%           
=======================================
  Files          16       16           
  Lines        3727     3744   +17     
  Branches      637      638    +1     
=======================================
+ Hits         3646     3663   +17     
  Misses         79       79           
  Partials        2        2           

Flag	Coverage Δ
#test262	54.77% <89.28%> (+0.26%)	⬆️
#tests	92.83% <100.00%> (+0.03%)	⬆️

Impacted Files	Coverage Δ
polyfill/lib/ecmascript.mjs	97.78% <100.00%> (+<0.01%)	⬆️
polyfill/lib/monthday.mjs	99.11% <100.00%> (+0.04%)	⬆️
polyfill/lib/slots.mjs	100.00% <100.00%> (ø)
polyfill/lib/yearmonth.mjs	97.39% <100.00%> (+0.08%)	⬆️

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[sffc]

Thanks! I think we'll also need to add the refYear/refDay to getISOFields() (#624), or else custom calendars won't be able to access that data.

[ryzokuken]

LGTM with just a few minor comments and Shane's comments addressed.

[ptomato]

Thanks for the reviews, I think all the comments are addressed. Good question about the maximum range, but I currently think the way the PR addressed it is the preferable way. I'll leave this sit for a bit while I work on other things, and come back and merge it later if no objection.

[sffc]

The only real requirement is that the year be a leap year. 1972 is as good as any other leap year. I think it's fine to pick one and make it hard coded. I had also suggested 2000 as a nice round number that is a leap year (not all years mod 100 are leap years).

[ptomato]

I mean, it's still hardcoded, just now with some justification 😄