init nix flake

the flake.nix has been initialized with:

```
nix flake init -t github:ipetkov/crane#quick-start-simple
```

This allows to build the project with

```
nix build
```

Which is useful to integrate the plugin in nix systems

flake.nix

@@ -0,0 +1,68 @@
+{
+  description = "Build a cargo project without extra checks";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+
+    crane = {
+      url = "github:ipetkov/crane";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, crane, flake-utils, ... }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = nixpkgs.legacyPackages.${system};
+
+        craneLib = crane.mkLib pkgs;
+
+        # Common arguments can be set here to avoid repeating them later
+        # Note: changes here will rebuild all dependency crates
+        commonArgs = {
+          src = craneLib.cleanCargoSource ./.;
+          strictDeps = true;
+
+          buildInputs = [
+            # Add additional build inputs here
+          ] ++ pkgs.lib.optionals pkgs.stdenv.isDarwin [
+            # Additional darwin specific inputs can be set here
+            pkgs.libiconv
+          ];
+        };
+
+        my-crate = craneLib.buildPackage (commonArgs // {
+          cargoArtifacts = craneLib.buildDepsOnly commonArgs;
+
+          # Additional environment variables or build phases/hooks can be set
+          # here *without* rebuilding all dependency crates
+          # MY_CUSTOM_VAR = "some value";
+        });
+      in
+      {
+        checks = {
+          inherit my-crate;
+        };
+
+        packages.default = my-crate;
+
+        apps.default = flake-utils.lib.mkApp {
+          drv = my-crate;
+        };
+
+        devShells.default = craneLib.devShell {
+          # Inherit inputs from checks.
+          checks = self.checks.${system};
+
+          # Additional dev-shell environment variables can be set directly
+          # MY_CUSTOM_DEVELOPMENT_VAR = "something else";
+
+          # Extra inputs can be added here; cargo and rustc are provided by default.
+          packages = [
+            # pkgs.ripgrep
+          ];
+        };
+      });
+}