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/horsea.asm

@@ -0,0 +1,10 @@
+; HorseaPokedexEntry: ; 0x1b9553
+	db "DRAGON@" ; species name
+	dw 104, 180 ; height, weight
+
+	db   "When they're in a"
+	next "safe location,"
+	next "they can be seen"
+	page "playfully tangling"
+	next "their tails"
+	next "together.@"