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

Move some data into a pokemon/ subdirectory

data/pokemon/dex_entries/onix.asm

@@ -0,0 +1,10 @@
+; OnixPokedexEntry: ; 0x1b8c98
+	db "ROCK SNAKE@" ; species name
+	dw 2810, 4630 ; height, weight
+
+	db   "As it digs through"
+	next "the ground, it"
+	next "absorbs many hard"
+	page "objects. This is"
+	next "what makes its"
+	next "body so solid.@"