[whatwg/url] Double-encoded IDNA labels don't roundtrip (#603)

Consider `xn--xn---epa`. It appears that using the current **domain to Unicode** algorithm (as implemented by Node.js), this would get converted to `xn--é`. But applying **domain to ASCII** on `xn--é` would produce a Punycode decoding failure. It sounds like domain to Unicode (or even UTS 46) should return failure on such labels.

This is somewhat important because Firefox can create such double-encoded labels:

```js
» new URL('http://xn--��').hostname
❮ "xn--xn---epa"
```

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