feat($theme-default): external links in prev/next (close #1962)(#1984)

Author: Sun Haoran

packages/@vuepress/theme-default/components/PageNav.vue

@@ -3,11 +3,13 @@
     <p class="inner">
       <span v-if="prev" class="prev">
         ←
-        <router-link v-if="prev" class="prev" :to="prev.path">{{ prev.title || prev.path }}</router-link>
+        <a v-if="prev.type === 'external'" class="prev" :href="prev.path" target="_blank" rel='noopener noreferrer'>{{ prev.title || prev.path }}<OutboundLink /></a>
+        <router-link v-else class="prev" :to="prev.path">{{ prev.title || prev.path }}</router-link>
       </span>
 
       <span v-if="next" class="next">
-        <router-link v-if="next" :to="next.path">{{ next.title || next.path }}</router-link>
+        <a v-if="next.type === 'external'" :href="next.path" target="_blank" rel='noopener noreferrer'>{{ next.title || next.path }}<OutboundLink /></a>
+        <router-link v-else :to="next.path">{{ next.title || next.path }}</router-link>
         →
       </span>
     </p>
@@ -98,6 +100,7 @@ function flatten (items, res) {
   }
 }
 </script>
+
 <style lang="stylus">
 @require '../styles/wrapper.styl'
 

packages/docs/docs/guide/frontmatter.md

@@ -116,3 +116,17 @@ See: [Default Theme Config > Disable the Navbar](../theme/default-theme-config.m
 - Default: `undefined`
 
 See: [Default Theme Config > Sidebar](../theme/default-theme-config.md#sidebar).
+
+### prev
+
+- Type: `boolean|string`
+- Default: `undefined`
+
+See: [Default Theme Config > Prev / Next Links](../theme/default-theme-config.md#prev-next-links).
+
+### next
+
+- Type: `boolean|string`
+- Default: `undefined`
+
+See: [Default Theme Config > Prev / Next Links](../theme/default-theme-config.md#prev-next-links).

packages/docs/docs/zh/guide/frontmatter.md

@@ -115,3 +115,17 @@ meta:
 - 默认值: `undefined`
 
 参考: [默认值 Theme Config > Sidebar](../theme/default-theme-config.md#侧边栏).
+
+### prev
+
+- 类型: `boolean|string`
+- 默认值: `undefined`
+
+参考: [默认值 Theme Config > Prev / Next Links](../theme/default-theme-config.md#上-下一篇链接).
+
+### next
+
+- 类型: `boolean|string`
+- 默认值: `undefined`
+
+参考: [默认值 Theme Config > Prev / Next Links](../theme/default-theme-config.md#上-下一篇链接).