Support `slot` element as part of the virtual tree

Currently axe-core ignores `slot` elements and does not add them to the virtual tree. However we've come to understand that `slot` elements can use CSS [to become a visible element](https://github.com/dequelabs/axe-core/issues/4468) (by changing the `display` property) and can even be given a background color, can have be used as [the target of an IDREF ARIA attribute](https://github.com/dequelabs/axe-core/issues/4335), and should probably alert users of [incorrect use of ARIA attributes and role](https://www.w3.org/TR/html-aria/#el-slot).