Improve `JWT.verify_strict/3` typespec with error tuple

[doofyus]

Hey, thanks for the great lib! 🎉

I think there might be an issue with the typespec for the mentioned function.

Currently JOSE.JWT.verify_strict/3 returns, on some input, {:error, any()}. However, this is not shown in the typespec - this causes the dialyzer to fail.

I'm attaching a demo script to test it out:

Mix.install([
  {:jose, "~> 1.11"}
])

verify_strict = fn jwt ->
  """
  -----BEGIN PUBLIC KEY-----
  MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE0xZi5oUFw1oeX6bMdLqMeb2b/RwR
  1VQhERuHUtlXWDKIKBKead6BEu0ClXiwBHDoGmyEnXjmitdzDUNmCLCMNA==
  -----END PUBLIC KEY-----
  """
  |> JOSE.JWK.from_pem()
  |> JOSE.JWK.to_map()
  |> JOSE.JWT.verify_strict(["ES256"], jwt)
end

# example of error outputs not present in the typespec
[
  "FOO",
  "{\"name\": \"John Doe\", \"age\": 30}",
  "eyJhbGciOiJFUzI1NiIsInR5cCI6IkpXVCJ9.eyJpc3MiOiJmb28iLCJleHAiOjE1MTYyMzkwMjIsImlhdCI6MTUxNjIzOTAyMiwianRpIjoiYm9vIn0.5J6"
]
|> Enum.map(&verify_strict.(&1))
|> Enum.each(&IO.inspect(&1))

# {:error, {:badarg, ["FOO"]}}
# {:error, {:badarg, ["{\"name\": \"John Doe\", \"age\": 30}"]}}
# {:error, :unsupported_json_module}

This could fix #167, and partialy #164 too.