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

Move some data into a pokemon/ subdirectory

data/pokemon/dex_entries/kingdra.asm

@@ -0,0 +1,10 @@
+; KingdraPokedexEntry: ; 0x1d0fd1
+	db "DRAGON@" ; species name
+	dw 511, 3350 ; height, weight
+
+	db   "It stores energy"
+	next "by sleeping at"
+	next "underwater depths"
+	page "at which no other"
+	next "life forms can"
+	next "survive.@"