Add Nix Flake

This PR contains Nix flakes with:

quick-lint-js cli,
quick-lint-js vimPlugin,
neovim that has quick-lint-js both plugin and cli ready to work

Co-authored-by: strager <strager.nds@gmail.com>

flake.nix

@@ -0,0 +1,35 @@
+{
+  inputs.flake-utils.url = github:numtide/flake-utils;
+  outputs = { self, nixpkgs, flake-utils }:
+  flake-utils.lib.eachSystem flake-utils.lib.defaultSystems (system:
+  let pkgs = nixpkgs.legacyPackages.${system};
+  in rec {
+    # Runs with nix build .#quick-lint-js
+    packages.quick-lint-js = pkgs.callPackage ./dist/nix/quick-lint-js.nix { };
+    packages.vimPlugin = pkgs.vimUtils.buildVimPlugin {
+      name = "quick-lint-js";
+      src = plugin/vim/quick-lint-js.vim;
+      meta = packages.quick-lint-js.meta;
+    };
+
+    packages.simple-neovim-with-qljs = pkgs.neovim.override {
+      configure = {
+        customRC = ''
+          let $PATH = $PATH.":${packages.quick-lint-js}/bin"
+        '';
+        packages.myPackages = {
+          start = with pkgs.vimPlugins;[
+            # loaded on launch
+              ale
+              packages.vimPlugin 
+            ];
+            # manually loadable by calling `:packadd $plugin-name`
+            opt = [ ];
+        };
+      };
+    };
+    # Runs with nix build
+    packages.default = packages.quick-lint-js;
+
+  });
+}