ui: Query the current staged pallet with stage plt subcommands

[ethanjli]

Currently, stage is missing subcommands like stage ls-img (or stage ls-bun-img) for querying various aspects of a staged pallet bundle, but it includes subcommands like stage show-bun-depl for querying other aspects of a staged pallet bundle. It may be cleaner and more intuitive if we make a stage plt --bun=next subcommand, whose pallet-querying subcommands would have the same names as the pallet-querying subcommands of plt; and to keep a consistent set of pallet-querying subcommands between stage plt and [dev] plt. We should set the default value of the optional --bun/--bundle flag to be either current or next, and return an error if that name is not bound to a staged pallet bundle; I would suggest defaulting to next, since it's the first bundle shown in stage show if it exists.