"validator role" for View Runners

[arjun]

A roadmap item. From weekly discussion:

JG: Another idea: a VD validator
NR: We have the JS reference impl, could be incorporated there...
NR: Add a "validator role" for engines? Expansion of GG's work in IG publisher
JG: To support GG's "can I run this VD" use case and check for optional features in the spec, etc