[WIP] feat(NcRichContenteditable): add description block

[ShGKme]

☑️ Resolves

• Fix [BITV] 9.3.3.2/3.3 - Comment section: The input fields for creating and editing comments lack visual labels. (2) nextcloud/server#37087

🖼️ Screenshots

🚧 Tasks

• Discuss design
• Update layout, add contenteditable container and description block
• Proxy all attrs from the old root element to a new one
  • check if this is required
• Add description prop with no default
• Add #description slot with description prop as default value
• Add show-DefaultDescription that enables showing classic Write message, use "@" to mention someone, use ":" for emoji autocompletion …
• Change default placeholder from use "@" to blah-blah to Write a message
  • ? Or just remove default placeholder ?
• Add tests

🏁 Checklist

• ⛑️ Tests are included or are not applicable
• 📘 Component documentation has been extended, updated or is not applicable

[ShGKme]

@marcoambrosini what do you think about the design?

We could also do something like GitHub design with think dashed border, I think.

[marcoambrosini]

@ShGKme I think that this is too complicated for the purpose of the sidebar. I also think labels are not a good way to deliver user documentation

[ShGKme]

Closed for not in flavor of nextcloud/server#40294