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/seel.asm

@@ -0,0 +1,10 @@
+; SeelPokedexEntry: ; 0x1b88bf
+	db "SEA LION@" ; species name
+	dw 307, 1980 ; height, weight
+
+	db   "The light blue fur"
+	next "that covers it"
+	next "keeps it protected"
+	page "against the cold."
+	next "It loves iceberg-"
+	next "filled oceans.@"