[css-pseudo-4] Let ::details-content opened details correctly set expanded #13096

When a details element is forced open using :details-content (with content-visibility: visible; and height: auto !important; or similar), today, the details element is exposed as closed to assistive technology. It'd be great if it'd properly report that it's expanded when forced opened, reducing the need for writing and running JavaScript to make this very powerful UI pattern accessible.