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

Move some data into a pokemon/ subdirectory

data/pokemon/dex_entries/muk.asm

@@ -0,0 +1,10 @@
+; MukPokedexEntry: ; 0x1b89fc
+	db "SLUDGE@" ; species name
+	dw 311, 660 ; height, weight
+
+	db   "As it moves, a"
+	next "very strong poison"
+	next "leaks from it,"
+	page "making the ground"
+	next "there barren for"
+	next "three years.@"