selectedcontent element and other updates related to customizable select

The new customizable select updates have landed in the HTML specification, and with that landing ARIA in HTML is updating to account for the new `selectedcontent` element as well as a new rule for the `button` element when it is used as the button part to a customized `select`.

Essentially, when the selectedcontent element is correctly used as a child of the button part of a customizable select, then no ARIA roles or attributes can be used. Similarly, the button part of the customizable select also does not allow ARIA roles or attributes to be set.

However, if someone uses the selectedcontent element incorrectly outside of the context of the button part, then authors may correct for this misuse by specifying ARIA attributes without restriction, since in this invalid context it is merely a role=generic element.

A reduced [test case](https://w3c.github.io/html-aria/tests/selectedcontent.html) was created to illustrate the updated rules.