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

Move some data into a pokemon/ subdirectory

data/pokemon/dex_entries/pidgey.asm

@@ -0,0 +1,10 @@
+; PidgeyPokedexEntry: ; 0x181cfa
+	db "TINY BIRD@" ; species name
+	dw 100, 40 ; height, weight
+
+	db   "It rapidly flaps"
+	next "its wings in the"
+	next "grass, stirring up"
+	page "a dust cloud that"
+	next "drives insect prey"
+	next "out into the open.@"