feat(core): add Promise/async-await support

[usernane]

Problem

The library currently only supports callbacks, which feels dated in modern JavaScript development.

Suggestion

• Make send() return a Promise
• Allow await ajax.send() syntax
• Maintain backward compatibility with existing callback system

Example API

// New Promise-based usage
const response = await ajax.send();
console.log(response.jsonResponse);

// Or with .then()
ajax.send().then(response => {
  console.log(response.jsonResponse);
}).catch(err => {
  console.error(err);
});

Benefits

• Aligns with modern JS patterns
• Enables cleaner async/await code
• Better error handling with try/catch