Explainer for API changes for calendar support

[sffc]

I made a doc detailing my understanding of the state of the Temporal calendar design, following the 10+ hours of discussions this week at TC39.

@pipobscure @robpalme @apaprocki @spectranaut @caiolima

[codecov]

Codecov Report

Merging #287 into main will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #287      +/-   ##
==========================================
- Coverage   78.06%   78.03%   -0.03%     
==========================================
  Files          17       17              
  Lines        3410     3410              
  Branches      338      338              
==========================================
- Hits         2662     2661       -1     
  Misses        733      733              
- Partials       15       16       +1

Impacted Files	Coverage Δ
polyfill/lib/ecmascript.mjs	91.62% <0%> (-0.11%)	⬇️

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Comments resolved

[sffc]

Thanks for all the comments! I created issues for all of the open questions. Let me know if I missed any. I plan to merge this PR soon.

[sffc]

I have opened issues for outstanding issues. I'd like to keep remaining comments on this PR limited in scope to editorial issues. As soon as I have an LGTM I will merge the explainer into the repo, which should have all open questions labeled as such. Once the explainer is merged, we can contribute follow-on PRs as the open questions are addressed.

[littledan]

Sorry for missing the Temporal meeting and this thread, but I'm missing something here: What's the motivation for a programmable calendar API, as opposed to just using an implementation-defined set of strings for calendars, like Intl does?

I'm curious, is it possible to get at the PartialIsoCalendar and mess with it?

[sffc]

What's the motivation for a programmable calendar API, as opposed to just using an implementation-defined set of strings for calendars, like Intl does?

Follow-up: #298

I'm curious, is it possible to get at the PartialIsoCalendar and mess with it?

@pipobscure has an AI to make a polyfill for this from the Temporal monthly sync meeting this morning.

[littledan]

Sorry, by "mess with it" I didn't mean try it out in a polyfill, but rather monkey-patch it and change the behavior of other Temporal.DateTime instances.