This implements the adjustment to our behavior, bringing us closer to the other browsers
(although with a better result for examples like "colo[u]r", which we continue to treat
as one word rather than three).
Simple reftest included; for now, I've put it with our in-house tests rather than under
web-platform-tests because the exact behavior here is somewhat under-specified.
Depends on D173203
Differential Revision: https://phabricator.services.mozilla.com/D173204
