fix(create-app): the node in the svelte template is incorrectly mounted (#2947)

Author: ruibaby

packages/create-app/template-svelte-ts/src/main.ts

@@ -1,7 +1,7 @@
 import App from './App.svelte'
 
 const app = new App({
-  target: document.body
+  target: document.getElementById('app')
 })
 
 export default app

packages/create-app/template-svelte/src/main.js

@@ -1,7 +1,7 @@
 import App from './App.svelte'
 
 const app = new App({
-  target: document.body
+  target: document.getElementById('app')
 })
 
 export default app