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

Move some data into a pokemon/ subdirectory

data/pokemon/dex_entries/mareep.asm

@@ -0,0 +1,10 @@
+; MareepPokedexEntry: ; 0x1cd551
+	db "WOOL@" ; species name
+	dw 200, 170 ; height, weight
+
+	db   "It stores lots of"
+	next "air in its soft"
+	next "fur, allowing it"
+	page "to stay cool in"
+	next "summer and warm"
+	next "in winter.@"