Where possible, use built-in JSON instead of Jason

Note that it's not possible to replace any call where we encode and
pretty print, as JSON doesn't have such functionality.

lib/last_fm/api_impl.ex

@@ -133,7 +133,7 @@ defmodule LastFm.APIImpl do
 
     case Finch.request(req, LastFm.Finch, @request_opts) do
       {:ok, response} when response.status == 200 ->
-        Jason.decode(response.body)
+        JSON.decode(response.body)
         |> identify_body()
 
       other ->