xml_ancestors as akin to xml_parent() and equivalent to xml_find_all(., "ancestor::*")

[MichaelChirico]

This is an equivalent to #409 in the other direction. For consistency it would be nice to have such a functional equivalent.

FWIW, we are also finding xml_parent() much more performant than xml_find_{first,all}(x, "parent::*"):

r-lib/lintr#2718 (comment)

So presumably the same could be true of xml_ancestor().

[MichaelChirico]

xml_parent(n = 2) also fits in nicely with R users' experience with parent.frame(n=2), so maybe this could also just be built in to xml_parent() directly.