feat($theme-default): inform screen readers link opens in new tab/window

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

@@ -1,14 +1,15 @@
-<template functional>
+<template>
   <svg
     class="icon outbound"
     xmlns="http://www.w3.org/2000/svg"
-    aria-hidden="true"
+    aria-labelledby="outbound-link-title"
     x="0px"
     y="0px"
     viewBox="0 0 100 100"
     width="15"
     height="15"
   >
+    <title id="outbound-link-title">{{ 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"
@@ -20,6 +21,16 @@
   </svg>
 </template>
 
+<script>
+export default {
+  computed: {
+    openInNewWindowTitle () {
+      return this.$themeLocaleConfig.openNewWindowText || '(opens new window)'
+    }
+  }
+}
+</script>
+
 <style>
 .icon.outbound {
   color: #aaa;