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

Move some data into a pokemon/ subdirectory

data/pokemon/dex_entries/pichu.asm

@@ -0,0 +1,10 @@
+; PichuPokedexEntry: ; 0x1cd243
+	db "TINY MOUSE@" ; species name
+	dw 100, 40 ; height, weight
+
+	db   "It is unskilled at"
+	next "storing electric"
+	next "power. Any kind of"
+	page "shock causes it to"
+	next "discharge energy"
+	next "spontaneously.@"