Add option to disable "Dune Package Management is available" notification

[davesnx]

I would prefer to not see this notification each time I open the editor, do you think a setting or any other detection might make sense here?

[voodoos]

Also noticed this one recently, I think we should just remove it and have the Dune Pkg sandbox activate by default if there is no user configuration and dune pkg enabled exists with return code 0.

cc @PizieDust