Support Resend template variables when exporting to Resend

[burnmaniac]

Goals

1. Allow users to define template variables that get passed to Resend's Templates API
2. Automatically replace preview values with Resend's {{{VAR}}} syntax when uploading
3. Variables should appear in Resend dashboard after upload

Background

When you upload a template to Resend via the preview server, the resulting template has no variables defined - even if your component clearly uses props.

For example, this component:

export default function AWSVerifyEmail({ verificationCode }: AWSVerifyEmailProps) {
  return (
    // ... template using {verificationCode}
  );
}

AWSVerifyEmail.PreviewProps = {
  verificationCode: '596853',
};

When uploaded, Resend dashboard shows "No variables" because the HTML just contains the literal 596853 value, not {{{VERIFICATION_CODE}}}.
Resend's API supports a variables parameter when creating templates:

await resend.templates.create({
  name: 'my-template',
  html: '<p>Code: {{{VERIFICATION_CODE}}}</p>',
  variables: [{ key: 'VERIFICATION_CODE', type: 'string', fallbackValue: '596853' }],
});

But there's currently no way to use this from React Email.

Proposal

Add an optional ResendVariables static property to email components:

AWSVerifyEmail.ResendVariables = [
  { key: 'VERIFICATION_CODE', previewValue: '596853', type: 'string' },
];

When exporting to Resend:

1. Find/replace previewValue with {{{key}}} in the HTML
2. Pass the variables array to Resend's API

I've got a working implementation locally, but I hacked it a bit, as I just wanted to see if it's working. Perhaps I could open a PR so it can be reviewed, and with some guidance, we could have a full solution, in case this is the way to go.