Add nix flake (#1908)

Adds a flake I have been using for development on a Nix box, which I've
confirmed works to run pre-commit linting, correctly installs plugins,
can debug extension, etc.

## Checklist

- [-] I have added
[tests](https://www.cursorless.org/docs/contributing/test-case-recorder/)
- [-] I have updated the
[docs](https://github.com/cursorless-dev/cursorless/tree/main/docs) and
[cheatsheet](https://github.com/cursorless-dev/cursorless/tree/main/cursorless-talon/src/cheatsheet)
- [-] I have not broken the cheatsheet

Co-authored-by: Pokey Rule <755842+pokey@users.noreply.github.com>

Author: fidgetingbits

.envrc

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

.gitignore

@@ -42,3 +42,6 @@ next-env.d.ts
 
 # test subset config
 packages/test-harness/testSubsetGrep.properties
+
+# nix
+.direnv/

flake.lock

@@ -0,0 +1,56 @@
+{
+  description = "A Nix-flake-based development environment for Cursorless";
+
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+
+  outputs =
+    { self, nixpkgs }:
+    let
+      supportedSystems = [
+        "x86_64-linux"
+        "aarch64-linux"
+        "x86_64-darwin"
+        "aarch64-darwin"
+      ];
+      forEachSupportedSystem =
+        f: nixpkgs.lib.genAttrs supportedSystems (system: f { pkgs = import nixpkgs { inherit system; }; });
+      pythonVersion = builtins.replaceStrings [ "py" ] [
+        "python"
+      ] (nixpkgs.lib.importTOML ./pyproject.toml).tool.ruff.target-version;
+    in
+    {
+      devShells = forEachSupportedSystem (
+        { pkgs }:
+        {
+          default = pkgs.mkShell {
+            packages =
+              let
+                python = pkgs.${pythonVersion};
+                pythonPackages = pkgs."${pythonVersion}Packages";
+              in
+              [
+                pkgs.corepack
+                pkgs.vsce
+                # https://github.com/NixOS/nixpkgs/pull/251418
+                (pkgs.pre-commit.overrideAttrs (previousAttrs: {
+                  makeWrapperArgs = ''
+                    --set PYTHONPATH $PYTHONPATH
+                  '';
+                }))
+
+                python
+              ];
+            # To prevent weird broken non-interactive bash terminal
+            buildInputs = [ pkgs.bashInteractive ];
+            shellHook = ''
+              if [ ! -f .git/hooks/pre-commit ]; then
+                pre-commit install
+              fi
+
+              pnpm install
+            '';
+          };
+        }
+      );
+    };
+}