Follow up on #10334

It's unclear how to specify view-transition-group in terms of shadow trees.
Sure, we can make the <<custom-ident>> a tree-scoped reference, which means it can't reference names from a different scope. However, what about nearest keyword etc? Keywords are not usually "tree-scoped".

Since view-transition-name is tree-scoped and view-transition-group doesn't make much sense in isolation from it, suggesting that both idents and keywords in view-transition-group are tree-scoped, and this property always resolves to none inside a shadow-tree.

//cc @khushalsagar @vmpstr

See also #10529