extract nsc/cmd/store

[ripienaar]

nsc/cmd/store is used by various things like for example https://github.com/nats-io/nats-account-server/blob/master/server/store/nscstore.go, the problem here is that the core store features - which is a handy generic nsc store access tool - is intertwined with a load of CLI stuff.

When for example nats-account-server or anyone else who wants to interact with a NSC format store they pull in a huge pile of cli related dependencies via go mod which they do not ever use.

Would be great if this was a small contained usable library