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!