Support CSS4 selectors

[danvk]

Three years go by and there's another set of CSS Selectors available!

Not all of these are relevant to JSONSelect, but the Subject of a selector with Child combinator would be phenomenally useful. It would allow you to select a value whose child has some property without using :has():

.foo .bar .baz  # selects .baz
.foo !.bar .baz # selects .bar