feat($core): Add generator meta tag to ssr index.html template (#2133)

Author: kefranabg

packages/@vuepress/core/lib/client/index.ssr.html

@@ -5,6 +5,7 @@
     <meta name="viewport" content="width=device-width,initial-scale=1">
     <title>{{ title }}</title>
     <meta name="description" content="{{ description }}">
+    <meta name="generator" content="VuePress {{ version }}">
     {{{ userHeadTags }}}
     {{{ pageMeta }}}
     {{{ renderResourceHints() }}}

packages/@vuepress/core/lib/node/build/index.js

@@ -10,6 +10,7 @@ const createClientConfig = require('../webpack/createClientConfig')
 const createServerConfig = require('../webpack/createServerConfig')
 const { createBundleRenderer } = require('vue-server-renderer')
 const { normalizeHeadTag, applyUserWebpackConfig } = require('../util/index')
+const { version } = require('../../../package')
 
 /**
  * Expose Build Process Class.
@@ -147,7 +148,8 @@ module.exports = class Build extends EventEmitter {
       pageMeta,
       title: 'VuePress',
       lang: 'en',
-      description: ''
+      description: '',
+      version
     }
 
     let html