Add OSC 1.1 Xpath `//` address pattern operator

[orchetect]

As per OSC 1.1 proposal, §3.4:

OSC 1.1 inherits the path multiple-level wildcard- matching operator // from XPath.

This overcomes the limitation of the OSC 1.0 * operator that only matches up to / boundaries. It also gives concrete semantics to the string // in an address—which was previously not explicitly forbidden but may have resulted in inconsistent address handling due to ambiguity (e.g., in UNIX style path operations // is a no-op equivalent to /).