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

Move some data into a pokemon/ subdirectory

data/pokemon/dex_entries/nidorina.asm

@@ -0,0 +1,10 @@
+; NidorinaPokedexEntry: ; 0x1822b6
+	db "POISON PIN@" ; species name
+	dw 207, 440 ; height, weight
+
+	db   "It has a docile"
+	next "nature. If it is"
+	next "threatened with"
+	page "attack, it raises"
+	next "the barbs that are"
+	next "all over its body.@"