Re: [w3c/payment-handler] Registration example accesses `registration` before it is defined (#200)

Also, if we aren't going to simplify (for whatever reason) to just using `requestPermission`, then the return value from `navigator.permissions.query`, I believe, is actually a [PermissionStatus](https://w3c.github.io/permissions/#permissionstatus), which would mean we should be checking its `state` attribute, whereas `requestPermission` returns the `PermissionState` directly:

```js
button.addEventListener("click", async() => {
  const { registration } =
    await navigator.serviceWorker.register('/sw.js');
  if (!registration.paymentManager) {
    return; // not supported, so bail out.
  }

  const permission =
    await navigator.permissions.query({ name: "paymenthandler" });
  switch (permission.state) {
    case "denied":
      return;
    case "prompt":
      const result = await registration.paymentManager.requestPermission();
      if (result !== "granted") {
        return;
      }
      break;
  }

  // Excellent, we got it! Let's now set up the user's cards.
  await addInstruments(registration);
}, { once: true });
```

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