feat: add app.renderAsync() for promise-based view rendering

[Abdel-Monaam-Aouini]

Summary

• Add app.renderAsync(name, options) method that returns a Promise instead of using callbacks
• Enables async/await usage for view rendering

Usage

// Before (callback)
app.render('email', { name: 'Tobi' }, (err, html) => {
  if (err) return next(err);
  res.send(html);
});

// After (async/await)
const html = await app.renderAsync('email', { name: 'Tobi' });
res.send(html);