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

Move some data into a pokemon/ subdirectory

data/pokemon/dex_entries/steelix.asm

@@ -0,0 +1,10 @@
+; SteelixPokedexEntry: ; 0x1d067c
+	db "IRON SNAKE@" ; species name
+	dw 3002, 8820 ; height, weight
+
+	db   "The many small"
+	next "metal particles"
+	next "that cover this"
+	page "#MON's body"
+	next "reflect bright"
+	next "light well.@"