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

Move some data into a pokemon/ subdirectory

data/pokemon/dex_entries/chansey.asm

@@ -0,0 +1,10 @@
+; ChanseyPokedexEntry: ; 0x1b942e
+	db "EGG@" ; species name
+	dw 307, 760 ; height, weight
+
+	db   "People try to"
+	next "catch it for its"
+	next "extremely"
+	page "nutritious eggs,"
+	next "but it rarely can"
+	next "be found.@"