Add Prof. Oak post-game fight

This was surprisingly easy to add, albeit a bit awkward. I learned how to add trainer battles without headers this way, so I may go back to CeladonU and restructure with this in mind.

This commit adds a fully functioning Professor Oak fight, using the Rival party choice script as a launchpad. You can desynchronise the choices easily, just a few number changes. I used the S. S. Anne 2F and Route 22 scripts to hodgepodge it together.

Because of how I did this, Oak is fully re-fightable, taking inspiration from XY Serena's daily re-fights.

I tried to add a Yes/No prompt, but it would still pull you into the fight, presumably because it's calling a beforebattle script before the code. It's a bit of a hassle and my CeladonU code sucked, so I decided to leave it out for now.

It's intended to be unlocked once you enter the Hall of Fame, thus that change there. That hasn't been tested properly so uhhhh *shrug*

scripts/HallOfFame.asm

@@ -97,6 +97,9 @@ HallofFameRoomScript1:
 	ld a, HS_CERULEAN_CAVE_GUY
 	ld [wMissableObjectIndex], a
 	predef HideObject
+	ld a, HS_ROUTE_1_OAK
+	ld [wMissableObjectIndex], a
+	predef ShowObject
 	ld a, $2
 	ld [wHallOfFameCurScript], a
 	ret