This adds a new class `ElementReferencePseudoElement` which forwards
any accesses to the referenced element for "element-reference" pseudo
elements.
This allows us to, for instance:
- Access the underlying `ComputedProperties` in getComputedStyle, which
includes inline style.
- Store and access `CustomPropertyData`
- Access the layout node for "resolved value" computation
- Apply animations from `update_animated_style_if_needed`
We register these with the originating element when calling
`set_associated_shadow_host_pseudo_element`, this requires us to append
the element to the tree beforehand since we need to be able to get the
shadow host.
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.
