ARIA in HTML update for the label element

[ARIA in HTML](https://github.com/w3c/html-aria/pull/556) was updated today to clarify that when a `label` element is not associated with a labelable element, that authors may specify ARIA roles and `aria-*` attributes applicable to the specified role.

A [reduced test case](https://w3c.github.io/html-aria/tests/label-element.html) was created to demonstrate the updated allowances.

This change is being made because for the use case of a `label` element being used and not associated with a labelable element, there's no draw backs to allowing ARIA attributes to being defined. But, if the `label` element is implicitly or explicitly associated with a labelable element, assigning ARIA attributes can resulted in invalid nesting of roles, or can break the naming association, resulting in the labeled element no longer having an accessible name.