feat(queue): add request queuing and concurrency limiting

[usernane]

Problem

Cannot limit concurrent requests, which can overwhelm servers or hit rate limits.

Suggestion

Add optional request queue with configurable concurrency.

Example API

// Global setting
AJAXRequest.setMaxConcurrent(3); // Max 3 simultaneous requests

// Or per-instance
const ajax = new AJAXRequest({
  queue: true,
  maxConcurrent: 2
});

Use Cases

• Prevent overwhelming server with too many requests
• Respect API rate limits
• Better control over network usage