ci: add GitHub Actions workflow

[tedteddy]

Summary

This PR adds GitHub Actions CI workflow to replace Travis CI for better integration and reliability.

Features

Multi-version Node.js Testing

• Tests run on Node.js 18, 20, and 22
• Ensures compatibility across LTS and current versions

Three-stage Pipeline

1. Lint: Runs ESLint on code
2. Test: Runs tests with coverage on multiple Node versions
3. Build: Verifies project builds successfully

Benefits over Travis CI

• Native GitHub integration
• Better performance
• More generous free tier for open source
• Easier configuration and debugging
• Matrix builds for multiple Node versions

Files Added

• .github/workflows/ci.yml: Main CI workflow

Notes

• Codecov integration requires setting CODECOV_TOKEN secret
• Travis CI (.travis.yml) can be removed after this is merged and verified