popover=hint support

[callie-openai]

Is your feature request related to how popovers work in general?
No

Is your feature request related to how this polyfill works? Please describe.
Sort of, it's on adding support for a new part of the popover spec.

Describe the solution you'd like
Add support for popover=hint as described in whatwg/html#9778.

Describe alternatives you've considered
Use alternative library / wait for browser support

Additional context
This is still early days, but having the popover polyfill support this would mean we could start using it prior to the more wider rollout in the browser engines if it's deemed stable enough.

[jgerigmeyer]

It sounds like there's still some ongoing discussion in whatwg/html#9778 as to whether this will be implemented in browsers or not -- is that correct?