[2.x] [Webpack config] Code splitting fails for external npm modules

### Current Behavior

When using dynamic imports for an external NPM module (e.g., `import('sortablejs')`), the generated chunk URL is incorrect. Flarum attempts to load the file from `assets/forum/sortablejs.js` instead of the extension's directory (e.g., `assets/js/your-extension/forum/sortablejs.js`), resulting in a 404 error.

### Steps to Reproduce

1. In a Flarum extension, dynamically import an external npm package (e.g., `import('sortablejs')`).
2. Build the extension.
3. Trigger the dynamic import in the browser.
4. Check the network tab: Flarum tries to load `assets/forum/sortable.js`, which returns a 404.

### Expected Behavior

Flarum should load the chunk from the extension's own assets directory (e.g., `assets/js/your-extension/forum/sortablejs.js`).

### Screenshots

_No response_

### Environment

- Flarum version: 2.0.0-rc.1

### Output of `php flarum info`

_No response_

### Possible Solution

A workaround is to create a local wrapper file inside the extension's `src/` directory (e.g., `src/forum/wrapper.js` containing `import sortablejs from 'sortablejs'; export default sortablejs;`) and import that local file instead.

### Additional Context

_No response_