Rename all files in data/pokedex/entries/

Some people suggested this would be a good idea, and make management of
the files easier. I'm inclined to agree.

data/pokedex/entries/gengar.asm

@@ -0,0 +1,10 @@
+; GengarPokedexEntry: ; 0x1b8c25
+	db "SHADOW@" ; species name
+	dw 411, 890 ; height, weight
+
+	db   "Hiding in people's"
+	next "shadows at night,"
+	next "it absorbs their"
+	page "heat. The chill it"
+	next "causes makes the"
+	next "victims shake.@"