Add Cross-Platform Build and Release Support

[aleksanderdag]

Overview

This PR adds cross-platform build and release support, enabling compatibility across Windows, macOS, and Linux.

Changes

• Upgrade to net9.0 (needed for X-plat build).
• Updated .csproj files and dependencies to support cross-platform builds.
• Added necessary RuntimeIdentifiers for cross-platform compatibility.
• Added Encoding.RegisterProvider for encodings
• Added new Universial project using Avalonia for X-plat release support.
• Universial has additional informational dialogs for missing fields.

Testing

• Verified build process on Windows and macOS
• Verified release artifacts on Windows and macOS.
• Confirmed that the application runs as expected on both platforms.
• Confirmed dialog appears for missing fields.
• Confirmed NES Rom is converted to GCI on both platforms.
• Tested GCI on Animal Crossing NES.
• Ensured that release artifacts are generated correctly for each platform.