Add Quarto Wizard support

[jimjam-slam]

@mcanouil kindly dropped by quarto-svelte to add Quarto Wizard support to the extension today! It made me realise that similar support for Closeted would be really nice, especially given the number of CSS classes we expose.

I haven't had much time to devote to Closeread since my daughter came along, but I'm hoping to carve some proper time out to finish widget and video support, plus some small things. I'd be tempted to put this on the list too, as it seems like a pretty simple thing to implement!

[mcanouil]

I can easily make the PR to add the schema and some code snippets, but as I was saying Quarto Wizard assumes the options are scoped to avoid conflicts and ensure proper completion, hovering, etc.
The expectation is:

extensions:
  extension-name:
    ...

For closeread, I'm not sure there are "global" options, so it should be fine.