Add typescript declaration

```
[ts]
Could not find a declaration file for module 'extract-css-chunks-webpack-plugin'. '/node_modules/extract-css-chunks-webpack-plugin/dist/cjs.js' implicitly has an 'any' type.
  Try `npm install @types/extract-css-chunks-webpack-plugin` if it exists or add a new declaration (.d.ts) file containing `declare module 'extract-css-chunks-webpack-plugin';`
```