[css-selectors-4] Use actual types in nth pseudo class selectors

[pyoor]

The syntax for nth pseudo class selectors currently use a simplified syntax which later references actual types in the prose. It would be simpler and better for tooling which parses this data to use the actual types.

For example:
:nth-child(An+B [of S]? )

Would be represented as:
:nth-child(<an-b> [of <complex-real-selector-list>]? )

If this is acceptable I'd be happy to supply a patch.

See also #9473.

[cdoublev]

If this is acceptable I'd be happy to supply a patch.

Please do, see #9480.