stream mgmt: expose CANCEL_MOVE nicely

[philpennock]

Request from you-know-who via me: give nats stream a subcommand or option there-of to syntactically sugar the cancel a move:

nats req '$JS.API.ACCOUNT.STREAM.CANCEL_MOVE.${ACCT}.${STREAM}' ''

Tested and confirmed this works in both system and stream-owner accounts.