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

Move some data into a pokemon/ subdirectory

data/pokemon/dex_entries/doduo.asm

@@ -0,0 +1,10 @@
+; DoduoPokedexEntry: ; 0x1b87d7
+	db "TWIN BIRD@" ; species name
+	dw 407, 860 ; height, weight
+
+	db   "It lives on a"
+	next "grassy plain where"
+	next "it can see a long"
+	page "way. If it sees an"
+	next "enemy, it runs"
+	next "away at 60 mph.@"