multi-system support and package definition

[lukasl-dev]

Cleaned up the flake.nix to get rid of the hardcoded aarch64-darwin system. It now uses nix-systems so it's actually portable across Linux and other architectures.

I also added a proper package definition for oyo. This means you can build or run it directly without needing to enter a dev shell.

What's new

• Multi-system: Works on Linux and Darwin now.
• Package: Defined oyo (and default). It reads the version directly from Cargo.toml.
• DevShell: Now system-agnostic; kept bacon and pkg-config in there.
• Git: Added result to .gitignore.

Try it out

# run locally
nix run .

# run from github
nix run github:lukasl-dev/oyo

# build binary to ./result
nix build .

[ahkohd]

@lukasl-dev LGTM! Thanks for your contribution.