Allow pseudo elements for link-in-text-block

[WilcoFiers]

We recently came across an example where a pseudo element was used to create an underline for a link in text block. This was done with a pseudo element so that the line could be animated. Axe-core reported this as a violation, which it shouldn't.

I'm not sure we can properly detect whether a :before or :after pseudo element is visible, but we can at least detect that pseudo elements exist and set them as incomplete.