Cross-platform Nix/Slint/Rust template

[manuel-plavsic]

I have created a template for developing cross-platform Slint/Rust apps with Nix -> https://codeberg.org/plavsic/nix_slint_rust_template.

I also tested that it works with every platform: Linux, macOS, Android (through the emulator on both Linux and macOS; the emulator has hardware acceleration support enabled out of the box) and iOS (through the iOS simulator). It took quite some time and effort, since there were some challenges, but it was worth it! ;)

Feedback and PRs to further improve the template are welcome!