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/raichu.asm

@@ -0,0 +1,10 @@
+; RaichuPokedexEntry: ; 0x182117
+	db "MOUSE@" ; species name
+	dw 207, 660 ; height, weight
+
+	db   "If its electric"
+	next "pouches run empty,"
+	next "it raises its tail"
+	page "to gather electri-"
+	next "city from the"
+	next "atmosphere.@"