[Selectlist] Include `<hr>` as an allowed child of `<listbox>`

[YummyBacon5]

The hr element is allowed as a child with a normal select element. Its use is to create a separator.

But it seems that this is not included with selectlist.

I think that it should be allowed for use with the listbox slot/element.

Although, currently, a hr is not allowed as a child of an optgroup (https://github.com/whatwg/html/issues/9247), so should it be allowed as one here?