Unsubscribes the connection from a topic
Output stack: ``
UNSUBSCRIBE stops connected client to from receiving messages from a
topic it previously subscribed to.
PumpkinDB> "topic" SUBSCRIBE UNSUBSCRIBE
Runtime allocations necessary for the server
EmptyStack error if there is less than one item on the stack
empty_stack : [UNSUBSCRIBE] TRY UNWRAP 0x04 EQUAL?.