feat(request): add request cancellation (abort) support

## Problem
No way to cancel in-flight AJAX requests.

## Suggestion
Expose an `abort()` method that wraps `XMLHttpRequest.abort()`.

## Example API
```javascript
const request = ajax.send();
// Later...
ajax.abort(); // or request.abort();
```

## Use Cases
- User navigates away before request completes
- Search-as-you-type (cancel previous request when new input arrives)
- Timeout handling