add flake

Because `nix run github:urbit/hoon-language-server` is
kinda cool.

Author: midlyx-hatrys

flake.lock

@@ -0,0 +1,40 @@
+{
+  description = "LSP bridge for Hoon";
+
+  inputs = {
+    nixpkgs = "github:nixos/nixpkgs/nixpkgs-unstable";
+
+    parts = {
+      url = "github:hercules-ci/flake-parts";
+      inputs.nixpkgs-lib.follows = "nixpkgs";
+    };
+  };
+
+  outputs = inputs @ {
+    self
+    , nixpkgs
+    , parts
+  }: parts.lib.mkFlake { inherit inputs; } {
+    systems = ["x86_64-linux" "aarch64-linux" "aarch64-darwin" "x86_64-darwin"];
+
+    flake = {
+      overlays.default = nixpkgs.lib.composeManyExtensions [
+        (final: prev: {
+          hoon-language-server = final.callPackage ./. { };
+        })
+      ];
+    };
+
+    perSystem = { pkgs, system, ... }: {
+      imports = [
+        { config._module.args.pkgs = import nixpkgs { inherit system; overlays = [self.overlays.default]; }; }
+      ];
+
+      packages.default = pkgs.hoon-language-server;
+      apps.default = {
+        type = "app";
+        program = "${pkgs.hoon-language-server}/bin/hoon-language-server";
+      };
+    };
+  };
+}