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/malloc_size_of/lib.rs

@@ -711,16 +711,10 @@ where
             Component::Class(..) |
             Component::AttributeInNoNamespaceExists { .. } |
             Component::AttributeInNoNamespace { .. } |
-            Component::FirstChild |
-            Component::LastChild |
-            Component::OnlyChild |
             Component::Root |
             Component::Empty |
             Component::Scope |
             Component::Nth(..) |
-            Component::FirstOfType |
-            Component::LastOfType |
-            Component::OnlyOfType |
             Component::Host(None) => 0,
         }
     }