WiX MSI creates generic icon in start menu and on desktop

### Preflight Checklist

* [X] I have read the [contribution documentation](https://github.com/electron-userland/electron-forge/blob/master/CONTRIBUTING.md) for this project.
* [X] I agree to follow the [code of conduct](https://github.com/electron/electron/blob/master/CODE_OF_CONDUCT.md) that this project follows, as appropriate.
* [X] I have searched the issue tracker for a bug that matches the one I want to file, without success.

### Issue Details

* **Electron Forge Version:**
* 6.0.0-beta.53
* **Electron Version:**
* 10.1.3, 9.3.1
* **Operating System:**
* Windows 10 Build 18362
* **Last Known Working Electron Forge version:**:
* 6.0.0-beta.52 (at least for the start menu part, this one didn't generate a desktop icon at all)

### Expected Behavior
The icons in start menu and on desktop use the application's icon as specified in packager config.


### Actual Behavior
The icons in start menu and on desktop use a generic icon.


### To Reproduce
- Set an icon through packager config
- Set up the WiX MSI maker
- Create an MSI installer
- Install the application
- Check the desktop icon and icon in start menu

### Additional Information
What I found during my investigation:
- It seems there is a bug in the latest version of `electron-wix-msi` that prevents extraction of the app's icon from the packaged exe. See also felixrieseberg/electron-wix-msi#99.
- If you fix that extraction, the extracted icon will still be of low quality, resulting in a blurry desktop icon.

The good news is that the latest versions of `electron-wix-msi` also allow specifying a custom icon to use. This means we could do the following:
- Easy fix: Adjust the documentation for `MakerWixConfig` to include the `appIconPath` property. Including this in your maker-wix config _already works_ at the moment, only the doc is missing.
- Improvement: Ideally, we can use the icon path specified in packager config as a default for this `appIconPath` property.

Demo that this works, on my own app:
- Commit: https://github.com/mistermicheels/current-task/commit/11fc415e03f1c8764618955a7da8336ffb8bd844
- Before (installer in artifacts): https://github.com/mistermicheels/current-task/actions/runs/296342941
- After (installer in artifacts): https://github.com/mistermicheels/current-task/actions/runs/303097091