[a11y] Add semantics to the Expansible widget. #175907
Description
Use case
After #174981 lands, we will have onExpand and onCollapse actions available, which will enable expansion and collapse via a11y.
As discussed here:
#174981 (review)
#174981 (comment)
Proposal
Add expanded semantics flag alongside onExpand and onCollapse semantics actions to the Expansible widget.