Remove all code from main.asm (some labeled INCBINs, like out-of-context graphics, are still present)

engine/routines/ stores isolated out-of-context routines as individual files. It might be preferable later to append them to their related engine/ files in unique little SECTIONs, relying on the linkerscript to place them appropriately; or some other organization method. In the meantime, they're now easily findable apart from main.asm's other content.

engine/routines/updatebattlehuds.asm

@@ -0,0 +1,9 @@
+_UpdateBattleHUDs:
+	farcall DrawPlayerHUD
+	ld hl, PlayerHPPal
+	call SetHPPal
+	farcall DrawEnemyHUD
+	ld hl, EnemyHPPal
+	call SetHPPal
+	farcall FinishBattleAnim
+	ret