feat: Add nixos files

[enk-it]

Add flake.nix file for future nixos contributors

[akinomyoga]

Thanks for your contribution. I'm unfamiliar with Nix, so I cannot properly review this PR. You might better consider submitting this to NixOS/nixpkgs actually.

I asked ChatGPT several questions, and as a summary, ChatGPT says

Thanks for the contribution! I appreciate adding a flake.nix for Nix users. However, our project doesn't use CMake, so including it is misleading and pulls in unnecessary dependencies. Would you mind revising the flake to list just the minimal tools we actually use? The devShell should list only the tools actually needed, such as:

• stdenv.cc (for C++ compiler)
• gnumake (gawk)
• coreutils (for basic commands like mkdir, mv, chmod)
• gzip (for compressing man pages)
• glibcLocales (for the locale command)
• gnugrep (grep)

Also, the flake.nix references a package.nix file that does not exist, which will cause build failures. It would be better to remove or fix the reference to package.nix.

The current flake.nix hardcodes system = "x86_64-linux", but it would be better to support multiple platforms. Could you update it to avoid fixing the system, so contributors on macOS and other architectures can use it?