Investigate changing ID-based WAI-ARIA value types due to referenceTarget

[rahimabdi]

This issue carries over follow-up work from: #2474.

As part of referenceTarget, HTML adds definitions for "single element reference" and "set of element references" attributes, formalizing the concept of attributes that reference other elements via ID.

ARIA should investigate whether this would require any changes to the "Mapping WAI-ARIA Value types to languages" section and specifically, the "WAI-ARIA value type" column and how ARIA attributes are categorized. The WAI-ARIA value type is also used in each attribute's characteristics table (under "Value") so any changes made would need to be reflected there as well.

From @alice @spectranaut's comments in #247, it sounds like the ARIA value types should remain host language-agnostic however, the "ID reference" type (now "single element reference" in HTML) and "ID reference list" (now "set of element references" in HTML) may not accurately convey values that aren't necessarily IDs (e.g., ariaActiveDescendantElement is Element, and ariaDescribedByElements is FrozenArray<>).

[alice]

Just a quick note that the HTML spec changes are an as-yet unreviewed draft, and the terminology and even logic there is very much subject to change!

I think the point stands, though, that "ID reference" and "ID reference list" don't capture even the current status quo in HTML given the IDL attributes, and that ARIA may need a more neutral term for relationship attributes which create references between elements.