Add Constant for 3 Main Bag Pockets

[Kurausukun]

There is already a define for the total number of pockets (NUM_BAG_POCKETS), but there is not one for the number of pockets not including the TM case and berry pouch, which are separate menus from the 3 main pockets. I have created the constant NUM_BAG_POCKETS_NO_CASES to deal with this as there are various loops and structs that refer to this quantity. Feedback is welcome on the name, I'm not totally happy with it but couldn't think of anything better.