build: modernize package tooling

[jestjosh]

What changed

• Replaced the obsolete Babel 5, Karma, Travis, and Webpack 1-era setup with current Babel, ESLint, Webpack 5, jsdom, and Node built-in test runner.
• Moved React and prop-types into peer dependencies while keeping modern dev versions for validation.
• Updated tests to use current React rendering APIs.
• Added GitHub Actions CI and refreshed README examples for modern React.
• Updated the release build helper and verified CJS/ESM/UMD package outputs.

Why

The previous dependency graph could no longer install cleanly with modern npm because it mixed Webpack 1-era tooling with newer Karma packages. This keeps the library usable and publishable with current Node/npm tooling.

How to test

• npm test
• npm run build
• npm audit
• npm pack --dry-run
• CJS and UMD export smoke check

Impact

No intended runtime API change. The package metadata now explicitly treats react and prop-types as peer dependencies. The PR targets master because this repository does not currently have a main branch.