title: Truncate date string into just year and week
body: <p>I'm using Luxon to manipulate <code>Date</code>s in my code. My method returns a date like this:</p>
  <p><code>DateTime.utc(1982, 5, 25).toISOWeekDate() //=&gt; '1982-W21-2' </code></p>
  <p>I need to knock off the day part and have <code>'1982-W21'</code>instead.</p>
  <p>Not too familiar with js, how would I &quot;knock off&quot; the day part..</p>