Re: [whatwg/url] Consider adding a constructor to make a URL from parts (#354)

I am trying to write an immutable wrapper for `URL` because I want to avoid using mutable objects. Unfortunately I believe this is not possible without something like the proposed `URL.from` from above.

Instead of overriding a property on a class instance (i.e. mutation), the idea is we can construct a new URL object immutably:

```ts
const urlClass = new URL('https://foo.bar/a?b');

 const fromClass = (klass) => ({
    hash: klass.hash,
    host: klass.host,
    hostname: klass.hostname,
    href: klass.href,
    password: klass.password,
    pathname: klass.pathname,
    port: klass.port,
    protocol: klass.protocol,
    origin: klass.origin,
    searchParams: klass.searchParams,
    search: klass.search,
    username: klass.username,
});

const urlObject = fromClass(urlClass);

// Now we have an object, we can create new objects immutably using spread,
// and then override properties as we wish in order to manipulate the URL
const newUrlObject = {
    ...urlObject,
    pathname: '/c',
    searchParams: new SearchParams('?d'),
};
```

Now, I want to convert `newUrlObject` back to a `URL` class so I can stringify it back to a URL string ("https://foo.bar/c?d"), e.g.

```ts
const result = toClass(newUrlObject).toString(); // "https://foo.bar/c?d"
```

I imagined I could write a function `toClass` that converts my URL object back to a `URL` class. But it seems there is no straightforward way to write this function, without something like the proposed `URL.from`.

For now, I'm having to use [Node's legacy `url.format`](https://nodejs.org/dist/latest-v12.x/docs/api/url.html#url_url_format_urlobject) to convert my URL object to a string, but I would like to avoid using this legacy API.

---

@domenic In response to the following suggestion of yours:

> I suggest people just use the following:
> 
> ```js
> Object.assign(new URL("https://example.com/"), { ... pieces go here ... });
> ```

I believe this won't work for some properties which are read only (`origin` and `searchParams`), so I don't think this is a usable solution unfortunately.

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