Adds nix flake devshell

[arpd]

I've only tested this on aarch64 macos, but should also work on any linux environment. May have to add LD_LIBRARY_PATH in that case.

[arpd]

This PR adds nix develop support, providing a (hopefully..) fully functional environment to build the project with.

I've not included the flake.lock file, typically this is added automatically but I've explicitly left it out for now.

To use this, you can either:

• cd korangar; direnv allow if you use direnv
• nix flake -c $SHELL if you don't use direnv

[vE5li]

Also I just remembered, that we probably want to include openssl, pkg-config, and libpcap in the flake as well, since they are needed for cargo test --all-features

[arpd]

I've updated the PR as per your suggestions @vE5li You can use it either with direnv, or nix develop -c $SHELL and successfully build and run the project. It uses the rust-toolchain.toml version of the toolchain.

[vE5li]

Very nice work! :)