Feature request: support for an additional preload entry point

When using the recommended security settings where browser windows are created with the option

```
new BrowserWindow({
webPreferences: {
nodeIntegration: false
}
});
```
but you still need access to some specific node APIs, the standard approach is to use a separate entry point that is specified as a 'preload' script before the main renderer entry point runs. This runs with full node integration available, and is able to store functions into global scope that can be used by the main renderer script after it starts.

It would be useful if this script could also be processed via webpack (and typescript if the relevant module is installed), in addition to the main and renderer scripts.