Add support for macOS 26+ layered icons

[kdroidFilter]

Summary

• Port of JetBrains/compose-multiplatform#5451
• Adds layeredIconDir DSL property to macOS {} block for both JVM and native targets
• Compiles .icon directories into Assets.car via xcrun actool (requires actool >= 26.0)
• Embeds Assets.car in the .app bundle and sets CFBundleIconName in Info.plist
• Gracefully degrades with a warning if actool is missing or too old

Changed files

• PlatformSettings.kt — new layeredIconDir: DirectoryProperty
• MacAssetsTool.kt — new utility for actool compilation and version checking
• AbstractJPackageTask.kt — layered icon support for JVM packaging
• AbstractNativeMacApplicationPackageAppDirTask.kt — layered icon support for native packaging
• InfoPlistBuilder.kt — added CFBundleIconName plist key
• osUtils.kt — added plutil to MacUtils
• configureJvmApplication.kt / configureNativeApplication.kt — DSL wiring
• README.md — documentation with usage examples

Usage

nativeDistributions {
    macOS {
        iconFile.set(project.file("icons/MyApp.icns"))       // fallback
        layeredIconDir.set(project.file("icons/MyApp.icon")) // layered
    }
}

Test plan

• Verify build compiles successfully on all platforms
• Test on macOS with Xcode 26+ that Assets.car is generated and included in the .app bundle
• Test that builds without layeredIconDir set still work unchanged
• Test graceful degradation when actool < 26 or missing