fix($core): vuejs#2627 Dup ID violates HTML5-2

Author: adico1

packages/@vuepress/core/lib/client/components/OutboundLink.vue

@@ -2,6 +2,7 @@
   <svg
     class="icon outbound"
     xmlns="http://www.w3.org/2000/svg"
+    :aria-labelledby="rIdx"
     role="img"
     focusable="false"
     x="0px"
@@ -10,7 +11,7 @@
     width="15"
     height="15"
   >
-    <title>{{ openInNewWindowTitle }}</title>
+    <title :id="rIdx">{{ openInNewWindowTitle }}</title>
     <path
       fill="currentColor"
       d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"
@@ -24,6 +25,11 @@
 
 <script>
 export default {
+  data: function () {
+    return {
+      rIdx: 'outbound-link-title-' + Math.floor(Math.random() * 1000000)
+    }
+  },
   computed: {
     openInNewWindowTitle () {
       return this.$themeLocaleConfig.openNewWindowText || '(opens new window)'