Escape variant modifiers when used in selectors and at-rules

[thecrypticace]

Fixes #19268

[thecrypticace]

I should've checked Oxide but I believe we'd already decided that modifiers can't contain "special" chars as a restriction:

(notice how the first one doesn't have any kind of decoration — that means Oxide won't even see it)

So even if we escape these it won't get picked up anyway.