Add support for getSetCookie() method

[nfriedly]

Now that whatwg/fetch#1346 is landed, the fetch() spec has a new response.headers.getSetCookie() method for getting individual (non-combined) set-cookie headers.

We should detect the presence of getSetCookie() and call it when available.

Additionally the documentation should be cleaned up to clarify that splitCookiesString() is only necessary when working with older implementations.

[nfriedly]

Note: to anyone who's interested, I believe you can already use this method manually, e.g. something like this:

const response = await fetch(/*...*/);

const cookies = setCookie.parse(response.headers.getSetCookie());

console.log(cookies); // should be an array of cookies