add Nix flake

this will allow folks to run moccasin using
`nix run github:rektdeckard/moccasin`, as well as integrate it to their
Nix configs otherwise.

.envrc

@@ -0,0 +1 @@
+use flake

.gitignore

@@ -1 +1,2 @@
 /target
+.direnv

README.md

@@ -32,6 +32,10 @@ cd /usr/pkgsrc/news/moccasin
 make install
 ```
 
+### Nix
+
+moccasin is available as a Nix flake: `github:rektdeckard/moccasin`.
+
 ## Usage
 
 Since "moccasin" is hard to spell and has too many letters, the executable is just called `mcsn`.

flake.nix

@@ -0,0 +1,37 @@
+{
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+    naersk.url = "github:nix-community/naersk";
+  };
+
+  outputs =
+    {
+      self,
+      flake-utils,
+      naersk,
+      nixpkgs,
+    }:
+    flake-utils.lib.eachDefaultSystem (
+      system:
+      let
+        pkgs = (import nixpkgs) { inherit system; };
+        naersk' = pkgs.callPackage naersk { };
+      in
+      {
+        # For `nix build` & `nix run`:
+        defaultPackage = naersk'.buildPackage { src = ./.; };
+
+        # For `nix develop`:
+        devShell = pkgs.mkShell {
+          nativeBuildInputs = with pkgs; [
+            rustc
+            cargo
+            rust-analyzer
+            clippy
+            rustfmt
+          ];
+        };
+      }
+    );
+}