Add a11y-support check for nested ownership

```html

Axe-core flags these correctly as failures. What it does not do however is report that these are failures because owning descendants isn't widely supported. We should add a check that can distinguish between "no owner/owned element at all" and "an owner/owned element, but done in a way that isn't accessibility supported".

I don't yet have a proposal on how to do this, but I'm sure we can come up with something. The newly introduced `aria.getOwner()` could help with that, and should probably get a few "unsupported" feature flags so that we can make this configurable.