Normative: toSorted methods must be stable

[bakkot]

Fixes #3422. Also fixes a closely related issue where toSorted was incorrectly not permitted to have implementation-defined order using the default comparator when ToString is inconsistent.

The stability requirements of Array.prototype.sort and TypedArray.prototype.sort are documented only in the introductory paragraphs of those sections. The actual sorting, and its detailed requirements except for stability, are in SortIndexedProperties.

When the toSorted methods were introduced to Array and TypedArray they referenced only SortIndexedProperties, thus implicitly (and accidentally) omitting the stability requirement. This PR spells out the stability requirement more formally in SortIndexedProperties, which means it is enforced on toSorted as well.

[bakkot]

I've added another commit fixing a closely related issue, which is that SortIndexedProperties says the order is implementation-defined for sort, but not toSorted, when using the default comparison and ToString returns inconsistent results. The commit makes that apply to toSorted as well.

[syg]

lgtm. WDYT about removing the "sort must be stable" sentence in the description of Array.prototype.sort, now that it's pushed to this AO and shared by all callers?

[bakkot]

SGTM, done.

[acutmore]

Thanks! Sorry I missed this in the original.

[ljharb]

ac511f4 is the commit that will fix the IPR check that I included in this PR; I'm going to merge past that one failure to fix main.