NF: Counterbalancer object in PsychoJS

[TEParsons]

Adds Counterbalancer object (matching https://github.com/psychopy/psychopy/pull/5967) which makes calls to the Shelf to handle counterbalancing.

Bits which were beyond my JS skills I've added a ReferenceError so it's clear what needs implementing still

[TEParsons]

Task linked: CU-8692uzc0q CounterBalance Component for Builder