[url] URLSearchParams: calling .set() and .append() should return itself (#90)

Given a list of search params (strings) to be inserted into a URLSearchParams object, it would be useful if calling .set() and .append() would return the instance of URLSearchParams. Two reasons: 

 1. URLSearchParams, although not "map-like", is extremely close to an ES Map in look and feel. ES `Map()` returns itself when a developer calls `set()`. Similarly, so do the calls on ES Set(). It is surprising that URLSearchParams does not work the same.   
 2. Using `listOfURL.reduce()` (or other functions that operate on lists that expect an object in return) would benefit from this.   

Consider, this would allow the following code: 

```JS
function buildQuery(items) {
  const params = new URLSearchParams();
  items
    .map(item => item.url)
    // This is not great, requires unnecessary "{ }", ";" x2, and "return"  :(
    .reduce(url => {
       params.append();
       return params;
    }, params)
  return params.toString();
}
```

Could become much cleaner:

```
function buildQuery(listOfThings) {
  return items
    .map(listOfThings => listOfThings.url)
    .reduce(url => params.append("foo", url), new URLSearchParams())
    .toString();
}
```

//cc @domenic 

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/url/issues/90