Re: [whatwg/url] Support relative URLs (#531)

It continues to be annoying that this doesn't work...
```js
new URL('./foo/bar.json');  // TypeError: "./foo/bar.json" cannot be parsed as a URL
```

However, for those who need a solution today, here's my suggestion: use the `Request` class. It does support relative URLs and it even respects `document.baseURI` so it works correctly in the presence of a `<base>` tag!
```js
// Current URL: https://example.com
(new Request('./foo/bar.json')).url;  // => https://example.com/foo/bar.json
(new Request('https://hello.com/world')).url;  // => https://hello.com/world
```

We can combine `Request` and `URL` to get a functioning URL resolver that supports relative URLs:

```js
const resolveUrl = (from, to) => {
    return new URL(to || '', new Request(from).url);
};
```

```js
// Current URL: https://example.com
resolveUrl('./foo/bar.json');  // => https://example.com/foo/bar.json
resolveUrl('/api/', './foo/bar.json');  // => https://example.com/api/foo/bar.json
resolveUrl('https://hello.com/world/', 'baz');  // => https://hello.com/world/baz
resolveUrl('https://hello.com/world/', '/baz');  // => https://hello.com/baz
```

Depending on the `Request` class for this obviously isn't ideal, but also it still won't work in environments like Node where there's no base URL to resolve against. An ideal API would just return a partial URL if a base is not known.

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