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

Move some data into a pokemon/ subdirectory

data/pokemon/dex_entries/shellder.asm

@@ -0,0 +1,10 @@
+; ShellderPokedexEntry: ; 0x1b8a68
+	db "BIVALVE@" ; species name
+	dw 100, 90 ; height, weight
+
+	db   "Clamping on to an"
+	next "opponent reveals"
+	next "its vulnerable"
+	page "parts, so it uses"
+	next "this move only as"
+	next "a last resort.@"