style: Store values for :nth- selectors in dedicated struct NthSelectorData

To accomplish this, all :nth- Components were replaced with type Nth,
which uses NthSelectorData.

Using NthSelectorData will make it easier to add selector lists for :nth
selectors later on, but this change by itself shouldn't change any
behavior.

Differential Revision: https://phabricator.services.mozilla.com/D165831

components/style/stylist.rs

@@ -1921,10 +1921,7 @@ fn component_needs_revalidation(
         Component::FirstChild |
         Component::LastChild |
         Component::OnlyChild |
-        Component::NthChild(..) |
-        Component::NthLastChild(..) |
-        Component::NthOfType(..) |
-        Component::NthLastOfType(..) |
+        Component::Nth(..) |
         Component::FirstOfType |
         Component::LastOfType |
         Component::OnlyOfType => true,