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

Move some data into a pokemon/ subdirectory

data/pokemon/dex_entries/dewgong.asm

@@ -0,0 +1,10 @@
+; DewgongPokedexEntry: ; 0x1b8934
+	db "SEA LION@" ; species name
+	dw 507, 2650 ; height, weight
+
+	db   "It sleeps under"
+	next "shallow ocean"
+	next "waters during the"
+	page "day, then looks"
+	next "for food at night"
+	next "when it's cold.@"