The main motivation here is that the CSS Parser needs to know about
PageSelectorList so that we can parse one in
`CSSPageRule::set_selector_text()`. Including all of `CSSPageRule.h`
there would pull in a lot of other headers that aren't needed.
This is a bit under-specced, specifically there's no definition of
CSSMarginDescriptors so I've gone with CSSStyleProperties for now. Gets
us 17 WPT subtests.
Ideally we'd be able to share the code between page selectors and style
ones, but given how simple page selectors are, some code duplication is
the simpler option.
