## ⚠️ BREAKING CHANGE

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).

## Summary
This PR modernizes the build tooling to current standards:
- **Convert from JavaScript to TypeScript**
- **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)

## Changes

### TypeScript conversion
- ✅ 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
- ✅ Add return type annotations for all exported functions
- ✅ Fix typo in package.json description: "if a to colors" → "if two colors"

### ESLint improvements
- ✅ Update eslint.config.mjs to match other packages in cluster
- ✅ Add TypeScript-specific linting rules
- ✅ Add @typescript-eslint/eslint-plugin 7.18.0
- ✅ Add @typescript-eslint/parser 7.18.0
- ✅ Add eslint-config-prettier 10.1.8

### ESM conversion (⚠️ Breaking Change)
- ✅ 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

### Build system improvements
- ✅ Replace Babel with TypeScript compiler
- ✅ Replace @babel/register with tsx for running tests
- ✅ Create .mocharc.json to use tsx with --import flag
- ✅ Add skipLibCheck to tsconfig for better compatibility
- ✅ Add @types/node 22.10.5
- ✅ Add @types/mocha 10.0.10

### CI/CD improvements
- ✅ Update CI workflow to test on Node.js 20, 22, 24 (removing EOL Node 18)
- ✅ Add explicit build verification step to workflow

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

## Migration Guide

### For consumers using CommonJS (require):
```javascript
// ❌ No longer works
const { passesWcagAa } = require('passes-wcag')

// ✅ Option 1: Convert to ESM
import { passesWcagAa } from 'passes-wcag'

// ✅ Option 2: Use dynamic import
const { passesWcagAa } = await import('passes-wcag')
```

### For consumers already using ESM:
No changes needed - the package now properly exports ESM with full TypeScript types.

## Test plan
- [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>