Split items/ and trainers/ into their data/ and engine/ components

Move some data into a pokemon/ subdirectory

data/pokemon/dex_entries/mantine.asm

@@ -0,0 +1,10 @@
+; MantinePokedexEntry: ; 0x1d0e25
+	db "KITE@" ; species name
+	dw 611, 4850 ; height, weight
+
+	db   "It swims along"
+	next "freely, eating"
+	next "things that swim"
+	page "into its mouth."
+	next "Its whole body is"
+	next "very coarse.@"