SHACL schema support

[gsvarovsky]

Use (a sub-/super-set of) SHACL to define constraints, in the domain data.

Originally posted by @gsvarovsky in m-ld/m-ld-spec#73 (comment)

Prioritised feature backlog:

• Predicate Paths
• Declaration of SHACL constraint extension
• sh:maxCount
• sh:minCount
• Documentation as top-level TOC entry
• Class-based Targets (sh:targetClass) 🚧 partially on edge (Node Shape usable for affected updates)
• sh:datatype
• sh:class
• other types of property path