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/gastly.asm

@@ -0,0 +1,10 @@
+; GastlyPokedexEntry: ; 0x1b8b4a
+	db "GAS@" ; species name
+	dw 403, 2 ; height, weight
+
+	db   "It wraps its op-"
+	next "ponent in its gas-"
+	next "like body, slowly"
+	page "weakening its prey"
+	next "by poisoning it"
+	next "through the skin.@"