feat: add new diagram command #116

[b9n2038]

Add command for create new diagram #116

[pomdtr]

Hey, sorry I missed this one!

I think the proper way to integrate a command like this is to integrate with the file/newFile menu. It would list the command in:

• the welcome page
• the Create: New File... command

see https://github.com/marp-team/marp-vscode/blob/main/package.json#L222 for an example.