Provide a way how to export HTML emails with various variable syntax

[jvorcak]

We'd like to support the following use case:

1. User comes to our platform - downloads the OSS theme we provide in react email
2. After downloading, she can adjust the corresponding components (footer links etc) and start using it in e.g. Next.js project
3. Some emails are usually sent from the authentication provider platforms (Supabase, Firebase, Auth0, etc). These platforms support different syntax flavours and variables. We should somehow make these templates easily exportable for these platforms.

I think our Preview Editor should support switching variable syntax in the HTML export mode.

cc @iamhectorsosa