fix: update svelte mount usage

Author: chenjiahan

rsbuild/svelte/package.json

@@ -1,6 +1,7 @@
 {
   "name": "rsbuild-svelte",
   "version": "1.0.0",
+  "type": "module",
   "private": true,
   "scripts": {
     "build": "rsbuild build",

rsbuild/svelte/src/index.ts

@@ -1,7 +1,8 @@
 import './index.css';
+import { mount } from 'svelte';
 import App from './App.svelte';
 
-const app = new App({
+const app = mount(App, {
   target: document.body,
   props: {
     name: 'world',