just found a way to add the external anchor image to links with target="_blank" via CSS:

a[target='_blank']::after {
  content: '';
  display: inline-block;
  width: 1em;
  height: 1em;
  margin-left: 0.25em;
  margin-right: 0.25em;
  background-size: 100%;
  background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewbox='0 0 12.2 12.2' width='14' height='14'%3E%3Cpath d='M5.7 0v1.5h4L4.5 6.7l1 1.1 5.3-5.2v3.9h1.4V0z'/%3E%3Cpath fill='none' d='M3.4 6.7l3-2.9H1.5v7h7V5.9l-3 2.9z'/%3E%3Cpath d='M8.5 5.9v4.9h-7v-7h4.9l1.5-1.6H0v10h10V4.4z'/%3E%3C/svg%3E");
}