This PR converts the package to **ESM-only** and **TypeScript**. Projects using `require()` will need to migrate to ESM or use dynamic imports.

After this PR CommonJS is no longer supported. See [ESM Module FAQ](https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c).

This PR modernizes the build tooling to current standards:
- **Convert from JavaScript to TypeScript**
- **ESLint 9** with flat config
- **ESM** module format (breaking change)
- **TypeScript 5.9.2**
- **tsx** for test execution
- **Node.js 20, 22, 24** support
- **get-contrast-ratio@1.0.0-1** (ESM version)
- **GitHub Actions** replacing Travis CI

- ✅ Convert src/index.js → src/index.ts with proper type annotations
- ✅ Convert test/test.js → test/test.ts
- ✅ Add TypeScript 5.9.2 with ES2022 target
- ✅ Create tsconfig.json
- ✅ Fix typo in JSDoc: "wich" → "which"

- ✅ Upgrade ESLint 6.6.0 → 9.32.0
- ✅ Create eslint.config.mjs with modern flat config format
- ✅ Remove old .eslintrc.json
- ✅ Add @eslint/eslintrc 3.3.1 and @eslint/js 9.32.0
- ✅ Add @typescript-eslint/eslint-plugin 7.18.0
- ✅ Add @typescript-eslint/parser 7.18.0
- ✅ Upgrade eslint-config-prettier to 10.1.8
- ✅ Upgrade eslint-plugin-import to 2.32.0

- ✅ Add "type": "module" to package.json
- ✅ Add exports field for proper ESM support
- ✅ Update tsconfig.json to output ES2022 modules
- ✅ Update test imports to use .js extension for ESM compatibility
- ✅ Update get-contrast-ratio dependency from 0.2.1 to 1.0.0-1
- ✅ Replace Babel with TypeScript compiler

- ✅ Replace Babel with TypeScript compiler
- ✅ Replace old Babel packages with modern tooling
- ✅ Replace @babel/register with tsx for running tests
- ✅ Create .mocharc.json to use tsx with --import flag
- ✅ Add skipLibCheck to tsconfig for better compatibility
- ✅ Upgrade Mocha 6.2.2 → 11.7.1
- ✅ Upgrade np 5.1.3 → 10.2.0
- ✅ Add @types/node 22.10.5
- ✅ Add @types/mocha 10.0.10

- ✅ Replace Travis CI with GitHub Actions
- ✅ Test on Node.js 20, 22, 24 (current supported versions)
- ✅ Use actions/checkout@v4 and actions/setup-node@v4
- ✅ Add explicit build verification step

- ✅ Tests now run against TypeScript source directly (faster iteration)
- ✅ Added test to verify built ESM output works correctly

```javascript
// ❌ No longer works
const getBestContrastColor = require('get-best-contrast-color')

// ✅ Option 1: Convert to ESM
import getBestContrastColor from 'get-best-contrast-color'

// ✅ Option 2: Use dynamic import
const { default: getBestContrastColor } = await import('get-best-contrast-color')
```

No changes needed - the package now properly exports ESM.

- [x] Build succeeds (yarn build)
- [x] Linting passes (yarn lint)
- [x] All tests pass (yarn test)
- [x] All 9 test cases passing (including new build output test)
- [x] Built ESM output verified to work correctly

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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