[feat]: add batch method

[caseybaggz]

Such a clean and great API of the resend client. I am in need of the send.batch API but am just now learning Rust so I don't trust myself in making it. As soon as it's published I'll happily support this project though:

import { Resend } from 'resend';

const resend = new Resend('re_123456789');

await resend.batch.send([
  {
    from: 'Acme <[email protected]>',
    to: ['[email protected]'],
    subject: 'hello world',
    html: '<h1>it works!</h1>',
  },
  {
    from: 'Acme <[email protected]>',
    to: ['[email protected]'],
    subject: 'world hello',
    html: '<p>it works!</p>',
  },
]);

[BadMask121]

i think you can do the same using vectors then call .send for each vector value of email you want to send