Use `aria-expanded` to toggle visibility

[jerivas]

https://github.com/oddbird/css-toggles/issues/10#issuecomment-1105571312

[scottaohara]

this would be what I'd expect for the "Accordion/disclosure components" examples.

And would likely be the expected property to expose for a variety (but not all) popups.

Additionally, similar to what I mention in issue #20, applying the roles directly to the current elements (re: overwriting their implicit ARIA semantics with button roles) negates the underlying structural semantics of these elements.

However, aria-expanded would not be expected for say an ARIA tab widget, where a tab role and aria-selected are the expectations for the roles and indicating which tab represents the currently available tabpanel.