Support anchoring on :hover

[jamesnw]

Currently, the polyfill does not support :hover selector on either the anchor or anchored element.

While there is no specific mention of :hover in the CSS spec, or any tests in the WPT, this does work in the Chromium implementation- https://codepen.io/jamessw/pen/zYVveBw

While making the anchored element conditional on :hover is likely to not be desired as it would cause a layout shift that likely un-hovers the element (and a loop), making the anchor conditional on :hover could be a useful pattern.