Add ActivityStreams vocabulary
Apparently there's a Turtle definition of the vocabulary after all!
Unfortunately it looks like SIOC can no longer be fetched: https://gitlab.com/vincenttunru/rdf-namespaces/-/jobs/712296358
But I'm guessing that ActivityStreams is more widely used than SIOC, and the old version of rdf-namespaces will still be available. If SIOC ever gets published as Turtle as well (I'm guessing that's the issue), then we can regenerate a new version.