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

Move some data into a pokemon/ subdirectory

data/pokemon/dex_entries/graveler.asm

@@ -0,0 +1,10 @@
+; GravelerPokedexEntry: ; 0x1b840c
+	db "ROCK@" ; species name
+	dw 303, 2320 ; height, weight
+
+	db   "It travels by rol-"
+	next "ling on mountain"
+	next "paths. If it gains"
+	page "too much speed, it"
+	next "stops by running"
+	next "into huge rocks.@"