`caravan` tool: report whether AtDepot caravan is ready to trade

[ab9rf]

an enhancement to the caravan plugin would be for caravans in the AtDepot state to indicate whether the caravan is "still unloading" or "ready to trade"

i'm putting this up as an issue instead of a PR because the "ready to trade" state is not immediately evident from the caravan state table

research reference: 0x14028f648 (50.09-steam) in function gamest__SCOPE__T_main_interface__SCOPE__T_trade::render

[myk002]

can borrow the logic from the "ready to trade" gui/notify notification