Custom widget rendering

[juliushaertl]

Research custom widgets so we can render our own elements to the whiteboard
https://github.com/excalidraw/excalidraw/pull/5164

This is required for implementing file embeddings (Office files, pdfs, video, audio) and possible future features

• iframe embedding seems possible
• excalidraw supports embedding files into the .excalidraw files
• decide if we embed the files or reference them in Nextcloud

[juliushaertl]

Lets start with integrating link previews with the existing embed functionality of excalidraw

https://docs.excalidraw.com/docs/@excalidraw/excalidraw/api/props/render-props#renderembeddable https://docs.excalidraw.com/docs/@excalidraw/excalidraw/api/props/#validateembeddable

We can use that to render our link previews https://github.com/nextcloud/text/blob/main/src/nodes/Preview.vue#L31-L33