url: add ToObject method to native URL class

Provides a factory method to convert a native URL class
into a JS URL object.

```c++
Environment* env = ...

URL url("http://example.org/a/b/c?query#fragment");

MaybeLocal<Value> val = url.ToObject(env);
```

PR-URL: https://github.com/nodejs/node/pull/12507


Reviewed-By: James M Snell <jasnell@gmail.com>