[fetch] Implement `Headers#getAll` and `Headers#getSetCookie()`

This matches Deno's behavior (get() combines, iterator preserves the order, set and append combine), but implements both the Cloudflare Workers `getAll()` and the potential standard `getSetCookie` function.  The rationale for choosing both is to better support libraries which check for `getAll` and also because `getSetCookie` seems a little confusing (names are hard)

This also makes `.toJSON` and JSON.stringify return an array for `Set-Cookie`