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

Move some data into a pokemon/ subdirectory

data/pokemon/dex_entries/pidgeotto.asm

@@ -0,0 +1,10 @@
+; PidgeottoPokedexEntry: ; 0x181d75
+	db "BIRD@" ; species name
+	dw 307, 660 ; height, weight
+
+	db   "It slowly flies in"
+	next "a circular pat-"
+	next "tern, all the"
+	page "while keeping a"
+	next "sharp lookout for"
+	next "prey.@"