feat: define __VUE_SSR__ flag

Author: yyx990803

packages/@vue/cli-service/lib/config/base.js

@@ -136,7 +136,8 @@ module.exports = (api, options) => {
         .plugin('feature-flags')
           .use(webpack.DefinePlugin, [{
             __VUE_OPTIONS_API__: 'true',
-            __VUE_PROD_DEVTOOLS__: 'false'
+            __VUE_PROD_DEVTOOLS__: 'false',
+            __VUE_SSR__: 'false'
           }])
     }