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/sandslash.asm

@@ -0,0 +1,10 @@
+; SandslashPokedexEntry: ; 0x1821e7
+	db "MOUSE@" ; species name
+	dw 303, 650 ; height, weight
+
+	db   "Adept at climbing"
+	next "trees, it rolls"
+	next "into a spiny ball,"
+	page "then attacks its"
+	next "enemies from"
+	next "above.@"