[css-scroll-anchoring-1] Anchor selection should use the flattened element tree

[eqrion]

Anchor selection works using a DOM tree traversal. This won't work with Shadow DOM as shadow trees are unreachable using the standard parent/child relationships.

I believe we should update the language to use the flattened element tree concept.

[eqrion]

cc @dbaron @theres-waldo @skobes