ARIA attributes for details summary polyfill

- Update details/summary polyfill so the correct ARIA attributes are being set
  Ensure aria-expanded is set on the summary element
  Ensure aria-hidden is set on the toggled content

- Update test page: /examples/details-summary/

Don't think the ternary operators are needed here and on the line below as `expanded` and `hidden` are booleans, which is what we need `aria-expanded` and `aria-hidden` to be set to.