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/ursaring.asm

@@ -0,0 +1,10 @@
+; UrsaringPokedexEntry: ; 0x1d0a41
+	db "HIBERNANT@" ; species name
+	dw 511, 2770 ; height, weight
+
+	db   "Although it has a"
+	next "large body, it is"
+	next "quite skilled at"
+	page "climbing trees. It"
+	next "eats and sleeps in"
+	next "the treetops.@"