perf: import mockXHR only in production (#2910)

* 自动排除mock

生产环境，使用webpack的条件编译自动排除mock数据

* perf: refine

Co-authored-by: 花裤衩 <panfree23@gmail.com>

src/main.js

@@ -25,11 +25,12 @@ import * as filters from './filters' // global filters
  * you can execute: mockXHR()
  *
  * Currently MockJs will be used in the production environment,
- * please remove it before going online! ! !
+ * please remove it before going online ! ! !
  */
-import { mockXHR } from '../mock'
 if (process.env.NODE_ENV === 'production') {
+  import('../mock').then(({ mockXHR }) => {
     mockXHR()
+  })
 }
 
 Vue.use(Element, {