add missing src/active.mjs from template

src/active.mjs

@@ -0,0 +1,16 @@
+/**
+ * Keeps the node active state in sync.
+ * @see {Router.updateActive}
+ * @param {Element} node
+ * @param {Router} router
+ */
+export function active(node, router) {
+  router.linkNodes.add(node);
+  router.updateActive(node);
+
+  return {
+    destroy() {
+      router.linkNodes.delete(node);
+    }
+  };
+}