Add testing infrastructure with Vitest and Testing Library

[matiasperz]

Summary

This PR adds comprehensive testing infrastructure to the library template, including Vitest configuration, Testing Library setup, example tests, and CI integration.

Key Changes

• Vitest Configuration (vitest.config.ts): Set up Vitest with jsdom environment for testing React components, global test APIs, and automatic test file discovery
• Test Setup (vitest.setup.ts): Configure jest-dom matchers for enhanced DOM assertions
• Example Tests: Added test files demonstrating testing patterns:
  • packages/core/core.test.tsx: React component and hook testing with Testing Library
  • packages/core/index.test.ts: Module export validation
• CI Workflow (.github/workflows/ci.yml): New GitHub Actions workflow that runs typecheck and tests on every PR and push to main
• Package Scripts: Added test and test:watch commands to package.json
• Dependencies: Added Vitest, Testing Library (React, DOM, jest-dom), jsdom, and react-dom as dev dependencies
• Documentation (README.md): Added testing section explaining the setup and how to run tests
• TypeScript Config: Updated tsconfig.json to include Vitest config files

Notable Details

• Tests run in a jsdom environment, enabling both logic and React component testing
• Test files follow the convention of living alongside source code as *.test.ts / *.test.tsx
• CI automatically validates code quality (typecheck) and test suite on every PR
• Setup is zero-config for developers—just run pnpm test or pnpm test:watch

https://claude.ai/code/session_01JZ7gpANXhJs9fkhrrLMz94