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

Move some data into a pokemon/ subdirectory

data/pokemon/dex_entries/forretress.asm

@@ -0,0 +1,10 @@
+; ForretressPokedexEntry: ; 0x1d0524
+	db "BAGWORM@" ; species name
+	dw 311, 2770 ; height, weight
+
+	db   "Usually found"
+	next "hanging on to a"
+	next "fat tree trunk. It"
+	page "shoots out bits of"
+	next "its shell when it"
+	next "sees action.@"