TypeError: generate is not a function

### Description

The `page-skeleton-webpack-plugin` was installed and configured using the project generated by the `@vue/cli 3.0` scaffold, but the skeleton screen could not be generated when the project was run.

使用`@vue/cli 3.0`脚手架生成的项目，安装并配置了`page-skeleton-webpack-plugin`，但是在运行项目时无法生成skeleton screen

### Steps to reproduce

1. Generate project using `vue/cli 3.0`
2. Install `page-skeleton-webpack-plugin` and configure it in `vue.config.js`
3. Execute npm run serve to run the project
4. Open the console and click on Preview skeleton page
5. An error has occurred 😭 -> "[PSG] generate is not a function"

-

1. 使用`vue/cli 3.0`生成项目
2. 安装`page-skeleton-webpack-plugin`，并在`vue.config.js`进行相应的配置
3. 执行npm run serve运行项目
4. 打开控制台，点击Preview skeleton page
5. 出现错误😭 -> "[PSG] generate is not a function"

**Expected behavior:**

Correct generation of skeleton screen.

正确的生成skeleton screen

**Actual behavior:**

An error message 😭 -> "[PSG] generate is not a function" has occurred and the debug skeleton screen cannot be previewed.

出现了错误提示😭 -> "[PSG] generate is not a function"，无法预览调试skeleton screen


### Plugin configration

```javascript
const path = require('path');
const { SkeletonPlugin } = require('page-skeleton-webpack-plugin');

module.exports = {
crossorigin: 'anonymous',
integrity: true,
configureWebpack: {
plugins: [
new SkeletonPlugin({
pathname: path.resolve(__dirname, './shell'),
staticDir: path.resolve(__dirname, './dist'),
routes: ['/'],
}),
],
},
};
```

### Versions

- Page Skeleton: 0.10.12
- Webpack: 4.17.1