Resend react prop: pass the component and the Node SDK renders it, and it is Node-only

# react prop: the SDK renders, you just pass the component

```tsx
import { Resend } from 'resend';
import { WelcomeEmail } from './emails/welcome';

const resend = new Resend(process.env.RESEND_API_KEY);

const { data, error } = await resend.emails.send({
  from: '[SENDER]',
  to: ['[RECIPIENT]'],
  subject: 'Welcome',
  react: WelcomeEmail({ name: '[NAME]' }),
});

if (error) {
  console.error(error);
}
```

## Checklist

- `react` takes the component. You do not need to render it to an HTML string
  yourself; the Node SDK does the HTML and plain-text rendering for you.
- The param is documented as only available in the Node.js SDK. Python, PHP,
  Ruby, and Go SDKs do not accept it; there you render to `html` yourself.
- Pick one body form per send: `react` or `html`. Passing the component means
  you do not also need an `html` string for the same email.
- The component must be a React Email component. Arbitrary JSX with
  browser-only APIs will not survive email rendering; keep it to email-safe
  markup.