优化 env demo 显示

src/pages/demo/playground/env/index.vue

@@ -1,19 +1,15 @@
 <template>
   <d2-container type="card">
     <template slot="header">process.env</template>
-    <p class="d2-mt-0">NODE_ENV = {{env}}</p>
-    <p>BASE_URL = {{baseUrl}}</p>
-    <p class="d2-mb-0">VUE_APP_TITLE = {{title}}</p>
+    <d2-highlight :code="env"/>
   </d2-container>
 </template>
 
 <script>
 export default {
   data () {
     return {
-      env: process.env.NODE_ENV,
-      baseUrl: process.env.BASE_URL,
-      title: process.env.VUE_APP_TITLE
+      env: JSON.stringify(process.env, null, 2)
     }
   }
 }