fix: add hint as a valid value for the popover attribute

[CaptainCodeman]

As per:

https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/popover#value

https://html.spec.whatwg.org/multipage/popover.html#attr-popover

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[Conduitry]

We shouldn't add this until it's no longer experimental. See #16082 (comment)

[CaptainCodeman]

Thanks - completely forgot about extending the element definitions.