fix: 修正 main.js 为 uni-app 3.x 写法

src/main.js

@@ -1,11 +1,9 @@
-import Vue from 'vue'
-import App from './App'
+import { createSSRApp } from 'vue'
+import App from './App.vue'
 
-Vue.config.productionTip = false
-
-App.mpType = 'page'
-
-const app = new Vue({
-  ...App
-})
-app.$mount()
+export function createApp() {
+  const app = createSSRApp(App)
+  return {
+    app
+  }
+}