Macros for keeping .partyFlags, .partySize, and .party in sync

[mrgriffin]

Introduces four macros for keeping .partyFlags, .partySize, and .party in sync, e.g.:

     [TRAINER_SAWYER_1] =
     {
-        .partyFlags = 0,
         .trainerClass = TRAINER_CLASS_HIKER,
         .encounterMusic_gender = TRAINER_ENCOUNTER_MUSIC_HIKER,
         .trainerPic = TRAINER_PIC_HIKER,
         .trainerName = _("SAWYER"),
         .items = {},
         .doubleBattle = FALSE,
         .aiFlags = AI_SCRIPT_CHECK_BAD_MOVE | AI_SCRIPT_TRY_TO_FAINT | AI_SCRIPT_CHECK_VIABILITY,
-        .partySize = ARRAY_COUNT(sParty_Sawyer1),
-        .party = {.NoItemDefaultMoves = sParty_Sawyer1},
+        .party = NO_ITEM_DEFAULT_MOVES(sParty_Sawyer1),
     },

The motivation is that new people often forget or don't know that they have to update .partyFlags and come to #pokeemerald asking about why their trainer's party is acting weirdly.

We had a quick discussion on Discord, and of the three replies I received at time of submitting this PR, they were neutral-to-positive on the .party = NO_ITEM_DEFAULT_MOVES(sParty_Sawyer1), syntax which is a bit gross, because we hide the initializations of .partySize and .partyFlags inside it; but I believe this design is better than having the macro also include .party because then that line wouldn't have a member name and that sticks out in a bad way.

For the benefit of people who find this PR because of a horrific merge conflict, I automated the changes in src/data/trainers.h with:

sed -i -e '/\.partyFlags/d' -e '/\.partySize/d' -e 's/{.NoItemDefaultMoves = \([A-Za-z0-9_]\+\)}/NO_ITEM_DEFAULT_MOVES(\1)/' -e 's/{.NoItemCustomMoves = \([A-Za-z0-9_]\+\)}/NO_ITEM_CUSTOM_MOVES(\1)/' -e 's/{.ItemDefaultMoves = \([A-Za-z0-9_]\+\)}/ITEM_DEFAULT_MOVES(\1)/' -e 's/{.ItemCustomMoves = \([A-Za-z0-9_]\+\)}/ITEM_CUSTOM_MOVES(\1)/' src/data/trainers.h