This will be used to look up a document's node navigable. It might be
nice to have a direct pointer from Document to its Navigable, but at
the moment I don't understand the algorithms well enough to maintain
that linkage.
This is the first step towards implementing the new "navigable" concept
from the HTML spec.
Co-authored-by: Aliaksandr Kalenik <kalenik.aliaksandr@gmail.com>