Add flake support

Have to use it with `nix run '.?submodules=1'`, unfortunately.

Might be able to remove that requirement by using the following, but I
don't know how it interacts with `gitignoreSource`:

```
fetchGit {
  url = ./.;
  submodules = true;
}
```

flake.nix

@@ -0,0 +1,43 @@
+{
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+
+    zig-overlay.url = "github:arqv/zig-overlay";
+    zig-overlay.inputs.nixpkgs.follows = "nixpkgs";
+
+    gitignore.url = "github:hercules-ci/gitignore.nix";
+    gitignore.inputs.nixpkgs.follows = "nixpkgs";
+  };
+  
+  outputs = {self, nixpkgs, zig-overlay, gitignore }:
+    let
+      inherit (gitignore.lib) gitignoreSource;
+      zls-derivation-fn = (system: nixpkgs.legacyPackages.${system}.stdenvNoCC.mkDerivation {
+        name = "zls";
+        version = "master";
+        src = gitignoreSource ./.;
+        nativeBuildInputs = [
+          zig-overlay.packages.${system}.master.latest
+        ];
+        dontConfigure = true;
+        dontInstall = true;
+        buildPhase = ''
+          mkdir -p $out
+          zig build install -Drelease-safe=true -Ddata_version=master --prefix $out
+        '';
+        XDG_CACHE_HOME = ".cache";
+      });
+    in {
+      packages = rec {
+        x86_64-linux.zls = zls-derivation-fn "x86_64-linux";
+        aarch64-linux.zls = zls-derivation-fn "aarch64-linux";
+        x86_64-darwin.zls = zls-derivation-fn "x86_64-darwin";
+        aarch64-darwin.zls = zls-derivation-fn "aarch64-darwin";
+
+        x86_64-linux.default = x86_64-linux.zls;
+        aarch64-linux.default = aarch64-linux.zls;
+        x86_64-darwin.default = x86_64-darwin.zls;
+        aarch64-darwin.default = aarch64-darwin.zls;
+      };
+  };
+}