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

Move some data into a pokemon/ subdirectory

data/pokemon/dex_entries/metapod.asm

@@ -0,0 +1,10 @@
+; MetapodPokedexEntry: ; 0x181ace
+	db "COCOON@" ; species name
+	dw 204, 220 ; height, weight
+
+	db   "This is its pre-"
+	next "evolved form. At"
+	next "this stage, it can"
+	page "only harden, so it"
+	next "remains motionless"
+	next "to avoid attack.@"