Schema(tron) for static template checking

[LandingEllipse]

An avenue to consider is implementing some of PT's state checks in schema(tron) form, to allow for in-editor feedback (for those who use schema-aware editors).

Because of the dynamic evaluation context of many of PT's properties, there are limited opportunities to delve deep into schematron territory, but we should be able to at least cover the basics (pt:sources needed in parent context of other properties, etc.).

The proposed static validator console script entry point could rely on this schema(tron) instead of implementing a separate backend in e.g. Template.