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/pikachu.asm

@@ -0,0 +1,10 @@
+; PikachuPokedexEntry: ; 0x1820ad
+	db "MOUSE@" ; species name
+	dw 104, 130 ; height, weight
+
+	db   "When it is anger-"
+	next "ed, it immediately"
+	next "discharges the"
+	page "energy stored in"
+	next "the pouches in its"
+	next "cheeks.@"