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.lock

@@ -0,0 +1,85 @@
+{
+  "nodes": {
+    "flake-utils": {
+      "locked": {
+        "lastModified": 1629481132,
+        "narHash": "sha256-JHgasjPR0/J1J3DRm4KxM4zTyAj4IOJY8vIl75v/kPI=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "997f7efcb746a9c140ce1f13c72263189225f482",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
+    "gitignore": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1657706534,
+        "narHash": "sha256-5jIzNHKtDu06mA325K/5CshUVb5r7sSmnRiula6Gr7o=",
+        "owner": "hercules-ci",
+        "repo": "gitignore.nix",
+        "rev": "f840a659d57e53fa751a9248b17149fd0cf2a221",
+        "type": "github"
+      },
+      "original": {
+        "owner": "hercules-ci",
+        "repo": "gitignore.nix",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1658119717,
+        "narHash": "sha256-4upOZIQQ7Bc4CprqnHsKnqYfw+arJeAuU+QcpjYBXW0=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "9eb60f25aff0d2218c848dd4574a0ab5e296cabe",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixpkgs-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "gitignore": "gitignore",
+        "nixpkgs": "nixpkgs",
+        "zig-overlay": "zig-overlay"
+      }
+    },
+    "zig-overlay": {
+      "inputs": {
+        "flake-utils": "flake-utils",
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1658105291,
+        "narHash": "sha256-3EXG2r5/h/iyzytRqI+tWTP11f1PdaJJ8Hl5QRe95OE=",
+        "owner": "arqv",
+        "repo": "zig-overlay",
+        "rev": "bfd640ef3c9bf8b4cd300af9f79a7ba950823ef5",
+        "type": "github"
+      },
+      "original": {
+        "owner": "arqv",
+        "repo": "zig-overlay",
+        "type": "github"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

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;
+      };
+  };
+}