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

Move some data into a pokemon/ subdirectory

data/pokemon/dex_entries/furret.asm

@@ -0,0 +1,10 @@
+; FurretPokedexEntry: ; 0x1ccddd
+	db "LONG BODY@" ; species name
+	dw 511, 720 ; height, weight
+
+	db   "It lives in narrow"
+	next "burrows that fit"
+	next "its slim body. The"
+	page "deeper the nests"
+	next "go, the more maze-"
+	next "like they become.@"