LibWeb: Support counter-* properties on pseudo-elements

There are multiple things happening here which are interconnected:

- We now use AbstractElement to refer to the source of a counter, which
  means we also need to pass that around to compute `content`.

- Give AbstractElement new helper methods that are needed by
  CountersSet, so it doesn't have to care whether it's dealing with a
  true Element or PseudoElement.

- The CountersSet algorithms now walk the layout tree instead of DOM
  tree, so TreeBuilder needs to wait until the layout node exists
  before it resolves counters for it.

- Resolve counters when creating a pseudo-element's layout node. We
  awkwardly compute the `content` value up to twice: Once to figure out
  what kind of node we need to make, and then if it's a string, we do
  so again after counters are resolved so we can get the true value of
  any `counter()` functions. This will need adjusting in the future but
  it works for now.

Libraries/LibWeb/DOM/PseudoElement.cpp

@@ -21,6 +21,8 @@ void PseudoElement::visit_edges(JS::Cell::Visitor& visitor)
     visitor.visit(m_cascaded_properties);
     visitor.visit(m_computed_properties);
     visitor.visit(m_layout_node);
+    if (m_counters_set)
+        m_counters_set->visit_edges(visitor);
 }
 
 Optional<CSS::CountersSet const&> PseudoElement::counters_set() const