Create a Nextcloud extension for Cristal

The goal is to provide a Nextcloud extension for Cristal.
The goal is to:

• inspire from https://github.com/manuelleduc/nextcloud-cristal but publish it as a proper contrib extension
• find out how to cleanly publish the extension in Nextcloud extension store
• identify limitations in Cristal preventing the extension to look good on Nextcloud (e.g., CSS issues)