## ⚠️ BREAKING CHANGE

This PR converts the package to **ESM-only**. 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:
- **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-relative-luminance@2.0.0-1** (ESM version)

## Changes

### ESLint 9 with flat config
- ✅ Upgrade ESLint 8.25.0 → 9.32.0
- ✅ Create `eslint.config.mjs` with modern flat config format
- ✅ Remove old `.eslintrc.json`
- ✅ Upgrade @typescript-eslint/eslint-plugin 5.62.0 → 7.18.0
- ✅ Upgrade @typescript-eslint/parser 5.62.0 → 7.18.0
- ✅ Upgrade eslint-config-prettier already at 10.1.8
- ✅ Upgrade eslint-plugin-import already at 2.32.0

### 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-relative-luminance dependency from 2.0.0-0 to 2.0.0-1

### Build system improvements
- ✅ TypeScript already at 5.9.2
- ✅ Replace ts-node with tsx for running tests
- ✅ Update .mocharc.json to use tsx with --import flag
- ✅ Add skipLibCheck to tsconfig for better compatibility
- ✅ Mocha already at 11.7.1
- ✅ Upgrade np 9.2.0 → 10.2.0
- ✅ Add @types/node for better type support
- ✅ Update @types/mocha 9.1.1 → 10.0.10

### CI/CD improvements
- ✅ Modernize GitHub Actions workflow
- ✅ Test on Node.js 20, 22, 24 (current supported versions)
- ✅ Update to actions/checkout@v4 and actions/setup-node@v4 (already done)
- ✅ Add explicit build verification step

### 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 getContrastRatio = require('get-contrast-ratio')

// ✅ Option 1: Convert to ESM
import getContrastRatio from 'get-contrast-ratio'

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

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

## Test plan
- [x] Build succeeds (`yarn build`)
- [x] Linting passes (`yarn lint`)
- [x] All tests pass (`yarn test`)
- [x] All 7 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>