title: Nanotime formatting with underscore separator for fractional seconds
body: <p>Working with nanoseconds from epoch is a breeze with <code>nanotime</code>, e.g.:</p>
  <pre><code>&gt; x  &lt;- 1639142792837258898
  &gt; xn &lt;- nanotime::nanotime(x)
  &gt; xn
  [1] 2021-12-10T22:26:32.837259008+09:00
  &gt; format(xn, format = &quot;%Y-%m-%dT%H:%M:%E9S&quot;)
  [1] &quot;2021-12-10T22:26:32.837259008&quot;
  </code></pre>
  <p><code>nanotime</code> also allows to use an underscore as separator for fractions of seconds <strong>when parsing</strong>, like so:</p>
  <pre><code>&gt; y &lt;- nanotime::as.nanotime(&quot;2021-12-10T22:26:32.837_259_008+09:00&quot;)
  &gt; y
  [1] 2021-12-10T22:26:32.837259008+09:00
  </code></pre>
  <p>How can I achieve the same style <strong>when formatting</strong>?</p>