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

Move some data into a pokemon/ subdirectory

data/pokemon/dex_entries/miltank.asm

@@ -0,0 +1,10 @@
+; MiltankPokedexEntry: ; 0x1d1482
+	db "MILK COW@" ; species name
+	dw 311, 1660 ; height, weight
+
+	db   "In order to milk a"
+	next "MILTANK, one must"
+	next "have a knack for"
+	page "rhythmically pull-"
+	next "ing up and down"
+	next "on its udders.@"