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

Move some data into a pokemon/ subdirectory

data/pokemon/dex_entries/tentacool.asm

@@ -0,0 +1,10 @@
+; TentacoolPokedexEntry: ; 0x1b82d1
+	db "JELLYFISH@" ; species name
+	dw 211, 1000 ; height, weight
+
+	db   "As it floats along"
+	next "on the waves, it"
+	next "uses its toxic"
+	page "feelers to stab"
+	next "anything it"
+	next "touches.@"