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/kurt_selectquantity_interpretjoypad.asm

@@ -0,0 +1,4 @@
+Kurt_SelectQuantity_InterpretJoypad: ; 27a28
+	call BuySellToss_InterpretJoypad
+	ld b, a
+	ret