Add wasm-pack flake for Rust

[winston0410]

I have a working flake for using wasm-pack in NixOS, but I am not sure if using rustc and rustup better than using rust-overlay like https://github.com/oxalica/rust-overlay. Any thoughts?

{
  description = "joli-string";

  outputs = { self, nixpkgs }:
    let system = "x86_64-linux";
    in {
      devShell.${system} = (({ pkgs, ... }:
        pkgs.mkShell {
          buildInputs = with pkgs; [
            wasm-pack
            cargo
            (rustc.override {
              stdenv = stdenv.override {
                targetPlatform = {
                  isRedox = false;
                  isMusl = false;
                  parsed = {
                    cpu = { name = "wasm32"; };
                    vendor = { name = "unknown"; };
                    kernel = { name = "unknown"; };
                    abi = { name = "unknown"; };
                  };
                };
              };
            })
            rustup
          ];

          shellHook = "";
        }) { pkgs = nixpkgs.legacyPackages.${system}; });
    };
}