Add 'drop' command to remove items from inventory

Players can ake items but cannot drop them. A drop command would allow placing inventory items back into the current room.

**Acceptance criteria:**
- Add CommandType.Drop to the enum
- CommandParser recognizes drop
- GameEngine.HandleDrop() removes item from inventory, adds to current room's RoomItems
- Non-droppable items (if any) should be handled gracefully
- Output confirmation message in green