UX: Temporarily add twitter-x Font Awesome icon (#22946)

This provides a pathway for a site to switch to the new X icon while we work on upgrading to FA6.

vendor/assets/svg-icons/discourse-additional.svg

@@ -56,4 +56,8 @@ Additional SVG icons
   <symbol id="discourse-threads" viewBox="0 0 16 17" fill-rule="evenodd" clip-rule="evenodd">
     <path d="M5 0L4.57143 3H1V5H4.28571L3.71429 9H0V11H3.42857L3 14L4.9799 14.2828L5.44888 11H7V9H5.73459L6.30602 5H11.2857L11 7H13.0203L13.306 5H16V3H13.5917L13.9799 0.282843L12 0L11.5714 3H6.59173L6.9799 0.282843L5 0ZM8 13.5V9C8 8.44772 8.44771 8 9 8H15C15.5523 8 16 8.44771 16 9V13.5C16 14.0523 15.5523 14.5 15 14.5H12.1194C11.5042 15.2014 10.396 16.3544 10.0417 16C9.97944 15.9223 9.99982 15.0667 10.0206 14.5H9C8.44771 14.5 8 14.0523 8 13.5Z"/>
   </symbol>
+  <!-- Temporary addition given it's a high-profile icon. Can be removed when we upgrade to Font Awesome 6.4.1 -->
+  <symbol id="temporary-twitter-x" viewBox="0 0 512 512">
+    <path d="M389.2 48h70.6L305.6 224.2 487 464H345L233.7 318.6 106.5 464H35.8L200.7 275.5 26.8 48H172.4L272.9 180.9 389.2 48zM364.4 421.8h39.1L151.1 88h-42L364.4 421.8z"/>
+  </symbol>
 </svg>