Parsing this pattern from CSS tokens turns out to be slightly crazy, but
thankfully well documented in the spec.
The spec lists the cases in order of simple -> complicated, but this
would cause problems in code, since `<n-dimension> <signed-.integer>`
would never by reached, as `<n-dimension>` comes before. Instead, I
have grouped them by their first token.
Also renamed the NthChildPattern class to ANPlusBPattern, to match spec
terminology.