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

Move some data into a pokemon/ subdirectory

data/pokemon/dex_entries/haunter.asm

@@ -0,0 +1,10 @@
+; HaunterPokedexEntry: ; 0x1b8bbd
+	db "GAS@" ; species name
+	dw 503, 2 ; height, weight
+
+	db   "It hides in the"
+	next "dark, planning to"
+	next "take the life of"
+	page "the next living"
+	next "thing that wanders"
+	next "close by.@"