Apparently there's a Turtle definition of the vocabulary after all!

Add ActivityStreams vocabulary

See merge request !7

Vocabularies that are unavailable during generation used to result
in index.ts not being generated. With this commit, the error is
logged, and the export is commented out in index.ts with a comment
explaining how it was unavailable when generating the exports.

If a vocabulary cannot be fetched, we wait for five seconds, then
try again. We do this at most five times per vocabulary.