Add Nix flake

Add a Nix flake that creates a package using `libgit2` and `pkg-config`.

Pinning nixpkgs to 22.11 ensures the correct version of `libgit2` is
used as per the README with no changed needed to the source code.

With this in place, the package can be build using `nix build` which
creates an executable at `result/bin/lite`.

A dev shell can also be started with `nix develop` based on the package
inputs.

Author: opdavies

flake.lock

@@ -0,0 +1,31 @@
+{
+  inputs = {
+    flake-parts.url = "github:hercules-ci/flake-parts";
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-22.11";
+  };
+
+  outputs = inputs:
+    inputs.flake-parts.lib.mkFlake { inherit inputs; } {
+      systems =
+        [ "x86_64-linux" "aarch64-linux" "aarch64-darwin" "x86_64-darwin" ];
+
+      perSystem = psArgs@{ pkgs, ... }: {
+        devShells.default =
+          pkgs.mkShell { inputsFrom = [ psArgs.self'.packages.default ]; };
+
+        packages.default = pkgs.buildGoModule {
+          pname = "splitsh-lite";
+          version = "2.0.0";
+          src = ./.;
+          vendorHash = "sha256-Hk08mULIpuPR7Wyyfpd4P0Vrcp9Cu0slUzZXhzK9t0M";
+
+          buildInputs = with pkgs; [ libgit2 ];
+          nativeBuildInputs = with pkgs; [ pkg-config ];
+
+          meta.mainProgram = "lite";
+        };
+
+        formatter = pkgs.nixfmt;
+      };
+    };
+}