Add flake.lock and flake.nix for package management and configuration

flake.nix

@@ -0,0 +1,53 @@
+{
+  inputs = {
+    opam-nix.url = "github:tweag/opam-nix";
+    flake-utils.url = "github:numtide/flake-utils";
+    nixpkgs.follows = "opam-nix/nixpkgs";
+  };
+  outputs = { self, flake-utils, opam-nix, nixpkgs }@inputs:
+    # Don't forget to put the package name instead of `throw':
+    let package = "tezla";
+    in flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = nixpkgs.legacyPackages.${system};
+        on = opam-nix.lib.${system};
+        devPackagesQuery = {
+          # You can add "development" packages here. They will get added to the devShell automatically.
+          ocaml-lsp-server = "*";
+          ocamlformat = "*";
+        };
+        query = devPackagesQuery // {
+          ## You can force versions of certain packages here, e.g:
+          ## - force the ocaml compiler to be taken from opam-repository:
+          ocaml-base-compiler = "*";
+          ## - or force the compiler to be taken from nixpkgs and be a certain version:
+          # ocaml-system = "4.14.0";
+          ## - or force ocamlfind to be a certain version:
+          # ocamlfind = "1.9.2";
+        };
+        scope = on.buildOpamProject { } package ./. query;
+        overlay = final: prev:
+          {
+            # Your overrides go here
+          };
+        scope' = scope.overrideScope overlay;
+        # The main package containing the executable
+        main = scope'.${package};
+        # Packages from devPackagesQuery
+        devPackages = builtins.attrValues
+          (pkgs.lib.getAttrs (builtins.attrNames devPackagesQuery) scope');
+      in
+      {
+        legacyPackages = scope';
+
+        packages.default = self.legacyPackages.${system}.${package};
+
+        devShells.default = pkgs.mkShell {
+          inputsFrom = [ main ];
+          buildInputs = devPackages ++ [
+            pkgs.nixd
+            # You can add packages from nixpkgs here
+          ];
+        };
+      });
+}