title: react-router v6 extending search params instead of replacing it
body: <p>Currently, doing <code>setSearchParams(/* an object */)</code> will replace the existing search params. If I would like to extend the search params instead of replacing them, e.g. adding a new param. What will be the most elegant way to achieve that?</p>
  <p>I know I can modify the <code>searchParams</code> directly, then do something like <code>setSearchParams(newSearchParamsToObject)</code>. Yet, since <code>searchParams</code> is not an object but a <code>URLSearchParams</code>. The code will be long and ugly.</p>
  <p>Edit: Since some one appears don't understand what I have posted</p>
  <p>Currently if we do something like this:</p>
  <pre><code>const [searchParams, setSearchParams] = useSearchParams({ a: 1 });
  setSearchParams({ b: 2 });
  </code></pre>
  <p>It will replace the <code>searchParams</code> with <code>?b=2</code>.</p>
  <p>What I would like to achieve is adding <code>b=2</code> to existing search params, i.e. the final search params should be <code>?a=1&amp;b=2</code>.</p>