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

Move some data into a pokemon/ subdirectory

data/pokemon/dex_entries/caterpie.asm

@@ -0,0 +1,10 @@
+; CaterpiePokedexEntry: ; 0x181a60
+	db "WORM@" ; species name
+	dw 100, 60 ; height, weight
+
+	db   "It crawls into"
+	next "foliage where it"
+	next "camouflages itself"
+	page "among leaves that"
+	next "are the same color"
+	next "as its body.@"