fix($theme-default): shouldn't transform date string at client side (close: #1035)

Move the transformation to $plugin-last-updated, BTW, expose `lang` as the 2nd argument

packages/@vuepress/plugin-last-updated/index.js

@@ -4,13 +4,20 @@ module.exports = (options = {}, context) => ({
   extendPageData ($page) {
     const { transformer } = options
     const timestamp = getGitLastUpdatedTimeStamp($page._filePath)
+    const $lang = $page._computed.$lang
     if (timestamp) {
-      const lastUpdated = typeof transformer === 'function' ? transformer(timestamp) : timestamp
+      const lastUpdated = typeof transformer === 'function'
+        ? transformer(timestamp, $lang)
+        : defaultTransformer(timestamp, $lang)
       $page.lastUpdated = lastUpdated
     }
   }
 })
 
+function defaultTransformer (timestamp, lang) {
+  return new Date(timestamp).toLocaleString(lang)
+}
+
 function getGitLastUpdatedTimeStamp (filePath) {
   let lastUpdated
   try {

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

@@ -73,9 +73,7 @@ export default {
     },
 
     lastUpdated () {
-      if (this.$page.lastUpdated) {
-        return new Date(this.$page.lastUpdated).toLocaleString(this.$lang)
-      }
+      return this.$page.lastUpdated
     },
 
     lastUpdatedText () {