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/updateitemdescription.asm

@@ -0,0 +1,13 @@
+UpdateItemDescription: ; 0x244c3
+	ld a, [MenuSelection]
+	ld [CurSpecies], a
+	hlcoord 0, 12
+	ld b, 4
+	ld c, SCREEN_WIDTH - 2
+	call TextBox
+	ld a, [MenuSelection]
+	cp -1
+	ret z
+	decoord 1, 14
+	farcall PrintItemDescription
+	ret