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

Move some data into a pokemon/ subdirectory

data/pokemon/dex_entries/slowbro.asm

@@ -0,0 +1,10 @@
+; SlowbroPokedexEntry: ; 0x1b8635
+	db "HERMITCRAB@" ; species name
+	dw 503, 1730 ; height, weight
+
+	db   "An attached"
+	next "SHELLDER won't let"
+	next "go because of the"
+	page "tasty flavor that"
+	next "oozes out of its"
+	next "tail.@"