[FR] A markup language for appflowy

### Description

Currently appflowy has two export option markdown and html .
Markdown has no widely accepted standard [example](https://gist.github.com/vimtaai/99f8c89e7d3d02a362117284684baa0f) .
Appflowy also has features that like gallery that none of the flavor's of markdown cover . Exporting as regular markdown is destructive . And for html it's not convenient either .

### Suggestion 1 : Allow to export as Wikitext/mediawiki markup

https://en.wikipedia.org/wiki/Help:Wikitext

Wikitext has way more formatting than markdown and can allow users to move or edit their note content in other apps and even in wikipedia (mediawiki)

### Suggestion 2 : A custom markup language for appflowy

If it is not possible to implement export/import as wikitext a modified markdown can be created for appflowy .

### Impact

1. Users can export data without losing formatting and blocks like Gallery , Image size , custom table width etc
2. Easy to import/export data
3. Easy to convert into other file formats using pandoc

### Additional Context

Pandoc support if custom markup is implemented.