feat(request): add timeout configuration

## Problem
Cannot configure request timeouts.

## Suggestion
Add `timeout` option to configuration and `setTimeout()` method.

## Example API
```javascript
const ajax = new AJAXRequest({
  url: '/api/data',
  timeout: 5000, // 5 seconds
  onTimeout: function() {
    console.log('Request timed out');
  }
});

// Or via method
ajax.setTimeout(5000);
```