style: Implement parsing and serialization for nth-child(An+B of selector list) and :nth-last-child(An+B of selector list)

:nth-{,last-}child parsing is disabled by default for now by pref
layout.css.nth-child-of.enabled.

Differential Revision: https://phabricator.services.mozilla.com/D165895

components/style/gecko/selector_parser.rs

@@ -318,6 +318,11 @@ impl<'a, 'i> ::selectors::Parser<'i> for SelectorParser<'a> {
         true
     }
 
+    #[inline]
+    fn parse_nth_child_of(&self) -> bool {
+        static_prefs::pref!("layout.css.nth-child-of.enabled")
+    }
+
     #[inline]
     fn parse_is_and_where(&self) -> bool {
         true

components/style/stylist.rs

@@ -1918,7 +1918,8 @@ fn component_needs_revalidation(
         Component::AttributeInNoNamespace { .. } |
         Component::AttributeOther(_) |
         Component::Empty |
-        Component::Nth(..) => true,
+        Component::Nth(_) |
+        Component::NthOf(_) => true,
         Component::NonTSPseudoClass(ref p) => p.needs_cache_revalidation(),
         _ => false,
     }