style: Represent FirstChild, FirstOfType, LastChild, LastOfType, OnlyChild, and OnlyOfType as functionless Nth variants

Like bug 1808226, this doesn't change any behavior, it just simplifies matching.

I also added a WPT JavaScript test for :only-of-type, since dedicated WPT
JavScript tests already exist for the other pseudo-classes this patch
touches.

Differential Revision: https://phabricator.services.mozilla.com/D165859

components/style/stylist.rs

@@ -1918,13 +1918,7 @@ fn component_needs_revalidation(
         Component::AttributeInNoNamespace { .. } |
         Component::AttributeOther(_) |
         Component::Empty |
-        Component::FirstChild |
-        Component::LastChild |
-        Component::OnlyChild |
-        Component::Nth(..) |
-        Component::FirstOfType |
-        Component::LastOfType |
-        Component::OnlyOfType => true,
+        Component::Nth(..) => true,
         Component::NonTSPseudoClass(ref p) => p.needs_cache_revalidation(),
         _ => false,
     }