Request to Implement Unsubscribe Functionality in the Mitt Framework

### Request to Implement Unsubscribe Functionality in the Mitt Framework

**Description:**
The current version of the Mitt framework lacks an `unsubscribe` feature, making it inconvenient to manage event listeners, especially when components are unmounted or no longer need to listen for events.

**Suggested Feature:**
Implement an `unsubscribe` method that allows users to easily remove specific event listeners.

**Example:**
```javascript
// Current usage in Mitt
const mitt = require('mitt');
const emitter = mitt();

const handler = () => console.log('Event triggered');
emitter.on('event', handler);
emitter.off('event', handler);
```
```javascript
// After adding the unsubscribe feature
const mitt = require('mitt');
const emitter = mitt();

// Proposed usage
const unsubscribe = emitter.on('event', () => console.log('Event triggered'));
unsubscribe(); // Now we can easily unsubscribe
```

**Conclusion:**
Adding the `unsubscribe` functionality would greatly improve the usability of the Mitt framework and align it with best practices in event handling. Thank you for considering this enhancement!

issue にはファイルやテストの指定がありません。Mitt の公開 emitter API から始め、既存の on/off の動作と提案されている戻り値を比較してください。互換性に関する想定を確認し、unsubscribe の使用方法に対するカバレッジを追加して、ドキュメントに記載された例が動作することを検証してください。

索引モデルが issue の本文から書いたものです。