chore: migrate toolchain to pnpm/vite/vitest (#50)

* docs: add toolchain migration design spec

Spec for migrating yarn→pnpm, microbundle→vite, jest→vitest.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* docs: update spec with review feedback

Address spec reviewer issues: consolidated vite.config.ts, added
outDir/husky v9/testing-library risk notes, added pnpm pack step.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* docs: add toolchain migration implementation plan

Step-by-step plan for yarn→pnpm, microbundle→vite, jest→vitest migration.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* chore: migrate from yarn to pnpm

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* chore: replace microbundle with vite for library builds

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* chore: replace jest with vitest

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* chore: update devDependencies to latest versions

* chore: remove design docs from codebase

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: use discriminated union for Action type

Replace `as T` assertions with proper type narrowing by making
Action<T> a discriminated union. TypeScript now knows newPresent
is non-optional in Set and Reset action types.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* docs: update install command to npm in README

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: homerchen19

.husky/pre-commit

@@ -1,4 +1,4 @@
 #!/bin/sh
 . "$(dirname "$0")/_/husky.sh"
 
-npm run pretty-quick
+pnpm run pretty-quick

README.md

@@ -14,7 +14,7 @@ undo/redo functionality with React [Hooks](https://reactjs.org/docs/hooks-intro.
 ## Installation
 
 ```sh
-yarn add use-undo
+npm install use-undo
 ```
 
 ## Usage

__test__/checkpoint.spec.tsx

@@ -1,4 +1,3 @@
-import React from 'react';
 import { render, cleanup, fireEvent } from '@testing-library/react';
 
 import useUndo from '../index';

__test__/index.spec.tsx

@@ -1,4 +1,3 @@
-import React from 'react';
 import { render, cleanup, fireEvent } from '@testing-library/react';
 
 import useUndo from '../index';

index.ts

@@ -16,11 +16,11 @@ export interface Actions<T> {
   canRedo: boolean;
 }
 
-interface Action<T> {
-  type: ActionType;
-  historyCheckpoint?: boolean;
-  newPresent?: T;
-}
+type Action<T> =
+  | { type: ActionType.Undo }
+  | { type: ActionType.Redo }
+  | { type: ActionType.Set; newPresent: T; historyCheckpoint?: boolean }
+  | { type: ActionType.Reset; newPresent: T };
 
 export interface State<T> {
   past: T[];

jest.config.js

@@ -12,41 +12,50 @@
   "bugs": {
     "url": "https://github.com/homerchen19/use-undo/issues"
   },
-  "source": "index.ts",
-  "main": "lib/use-undo.js",
-  "umd:main": "lib/use-undo.umd.js",
-  "module": "lib/use-undo.m.js",
+  "main": "lib/use-undo.cjs",
+  "module": "lib/use-undo.mjs",
   "types": "lib/index.d.ts",
+  "exports": {
+    ".": {
+      "import": {
+        "types": "./lib/index.d.ts",
+        "default": "./lib/use-undo.mjs"
+      },
+      "require": {
+        "types": "./lib/index.d.ts",
+        "default": "./lib/use-undo.cjs"
+      }
+    }
+  },
   "scripts": {
-    "build": "rimraf lib && microbundle -o lib/ --name use-undo --sourcemap false --no-compress",
-    "prepublishOnly": "yarn build",
-    "preversion": "yarn test:cov",
-    "test": "jest",
-    "test:cov": "jest --coverage --runInBand --forceExit",
-    "test:watch": "jest --watch",
-    "pre-commit": "pretty-quick --staged",
+    "build": "vite build",
+    "prepublishOnly": "pnpm build",
+    "preversion": "pnpm test:cov",
+    "test": "vitest run",
+    "test:cov": "vitest run --coverage",
+    "test:watch": "vitest",
     "pretty-quick": "pretty-quick",
-    "prepare": "husky install"
+    "prepare": "husky"
   },
   "files": [
     "lib"
   ],
   "devDependencies": {
-    "@testing-library/react": "^11.2.7",
-    "@types/jest": "^27",
-    "@types/node": "^15.6.1",
-    "@types/react": "^17.0.8",
-    "@types/react-dom": "^17.0.5",
-    "husky": "^6.0.0",
-    "jest": "^27.0.3",
-    "microbundle": "^0.13.1",
-    "prettier": "^2.3.0",
-    "pretty-quick": "^3.1.0",
-    "react": "^17.0.2",
-    "react-dom": "^17.0.2",
-    "rimraf": "^3.0.2",
-    "ts-jest": "^27.0.1",
-    "typescript": "^4.3.2"
+    "@testing-library/dom": "^10.4.1",
+    "@testing-library/react": "^16.3.2",
+    "@types/react": "^18.3.28",
+    "@types/react-dom": "^18.3.7",
+    "@vitest/coverage-v8": "^4.1.1",
+    "husky": "^9.1.7",
+    "jsdom": "^29.0.1",
+    "prettier": "^3.8.1",
+    "pretty-quick": "^4.2.2",
+    "react": "^18.3.1",
+    "react-dom": "^18.3.1",
+    "typescript": "^6.0.2",
+    "vite": "^8.0.2",
+    "vite-plugin-dts": "^4.5.4",
+    "vitest": "^4.1.1"
   },
   "peerDependencies": {
     "react": ">=16.8.6",