Define 'type' as a property on errors rather than a getter

[richardm-stripe]

cc @rattrayalex-stripe for discussion:

I don't have a full understanding of why it was originally important for 'type' to be implemented as a getter -- or if we should do this -- but this PR both

1. Passes the existing test/Error.spec.js
2. Fulfills the ask in StripeError.type missing from logs #718

      class Foo extends Error.StripeError {}
      const err = new Foo({message: 'hi'});
      console.log(err);

outputs

...
  {
  raw: { message: 'hi' },
  rawType: undefined,
  code: undefined,
  param: undefined,
  detail: undefined,
  headers: undefined,
  requestId: undefined,
  statusCode: undefined,
  charge: undefined,
  decline_code: undefined,
  payment_intent: undefined,
  payment_method: undefined,
  setup_intent: undefined,
  source: undefined,
  type: 'Foo'
}

and

      const Custom = Error.StripeError.extend({type: 'Foo'});
      const err = new Custom({message: 'hi'});
      console.log(err);

outputs the same.

[rattrayalex-stripe]

Nice, yep, this looks cleaner! And yes, type should definitely be enumerable; oops.

Thanks Richard!