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/mewtwo.asm

@@ -0,0 +1,10 @@
+; MewtwoPokedexEntry: ; 0x1cc8ae
+	db "GENETIC@" ; species name
+	dw 607, 2690 ; height, weight
+
+	db   "Said to rest qui-"
+	next "etly in an"
+	next "undiscovered cave,"
+	page "this #MON was"
+	next "created solely for"
+	next "battling.@"