Electron-forge: packagerConfig icon property that will work with both windows and osx

[jasonok6]

Preflight Checklist

• [x] I have read the contribution documentation for this project.
• [x] I agree to follow the code of conduct that this project follows, as appropriate.
• [x] I have searched the issue tracker for a feature request that matches the one I want to file, without success.

Problem Description

Currently I have to change the packagerConfig.icon property manually depending on what platform I am building for. osx requires a icns file and windows requires a .ico file. It would
be nice if there were a solution to build against both platforms without having to manually alter the config file.

Proposed Solution

A config solution that will allow building either a mac or windows app without having to manually change the packagerConfig.icon property

[malept]

Per the Packager icon documentation:

If the file extension is omitted, it is auto-completed to the correct extension based on the platform