Add a new functionality to scarpet to list all items in a category (like weapons or building_blocks). This could be added to item_list(tag?), which queried with a category instead of a tag lists all items in said category, to item_category(item?), with the same idea, or to a new function item_category_list(category?), which would return the list of available categories when not given an argument, or list the items in the given category.

technically you could just create your own list statically using item_list and item_category. See no reason to add extra API method just for that.