Use of `"@vocab": "_:"` has been deprecated since JSON-LD 1.0

offshoot of #675

context

between the "CG Final Report" and "1.0" of JSON-LD: https://www.w3.org/TR/json-ld11/#changes-from-cg

The use of blank node identifiers to label properties is obsolete, and may be removed in a future version of JSON-LD, as is the support for generalized RDF Datasets.

our normative context document https://www.w3.org/ns/activitystreams.jsonld contains this:

"@vocab": "_:"

this is the deprecated behavior mentioned in JSON-LD.

what to do

• Needs Group Input/Discussion: should we keep this deprecated behavior? is anyone relying on parsing AS2 documents as generalized rdf, or are they simply ignoring undefined properties/predicates?
  • Next Version: AS2.1 could describe what to do when a property isn't defined. it could also describe how to define a property properly -- we currently refer to such properties as "extension properties", but we don't provide much if any guidance on how to work with them.
    • Needs Primer: whatever information makes it into AS2.1 should also be included in the primer
    • also update the JSON-LD context document to reflect whatever the decision is

Start with issue #675, the JSON-LD 1.1 changes section, and the normative activitystreams.jsonld context. The group must decide whether to retain the deprecated @vocab behavior and how AS2.1 should handle undefined properties. Done means the decision is reflected in AS2.1, the primer, and the JSON-LD context document.