We do plan to support a bunch of extensions soon. We do support pg_vector at this point. Would love to know if you have any favorite ones. Also, could you share the use case for listen/notify? We currently do not support it but have plans for something that gives you the same functionality.
As far as extensions I think postgis should be pretty much a default in all PG installations these days.
I use listen/notify and triggers as some sort of pubsub.