The parsing of :where and :is is not spec compliant

The parsing for `:is` and `:where` currently implements parsing a list of compound selectors while CSS supports any selector lists, i.e. lists of complex selectors.

Side note: the naming is confusing in this package as it uses the name "simple selector" to actually represent what is called "compound selector" in the CSS spec: https://www.w3.org/TR/selectors-4/#structure