The tricky bit of this is resolving cycles in extending rules and
ensuring that counter styles are registered in the required order for
extension (i.e. for any pair of extended/extending rules the extended
one should be registered first).
This serves as an intermediate step between `CSSCounterStyleRule` and a
fully built counter style which will be helpful for:
- Determining whether a rule defines a counter style
- Resolving extends cycles
- Determining registration order
