Support Icon Composer files from macOS26

[tkomde]

Electron packager appears to support Icon Composer files introduced in macOS 26 with version 18.4.0.
electron/packager#1806

Even if you try to use this with electron-forge, the current documentation doesn't describe how to do it. It seems like making the following changes should work.

| macOS            | `.icns` | 512x512 pixels (1024x1024 for Retina displays) |

->

| macOS            | `.icns` or '.icon' | 512x512 pixels (1024x1024 for Retina displays) |

  packagerConfig: {
    icon: '/path/to/icon' // no file extension required
  }

->

  packagerConfig: { 
    // Traditional way
    icon: '/path/to/icon' // no file extension required
    // To support Icon Composer files from macOS 26, uncomment following.
    // (Requires Xcode 26 is installed and actool is available.)
    // icon: process.platform === "darwin" ? ["./assets/icon.icns", "./assets/icon.icon"] : "./assets/icon"
  }

While it is possible to create a PR, I will raise it as an issue for now.