Add support for new Scroll View mode of Revealjs

[cderv]

This is a new feature from 1.5 : https://revealjs.com/scroll-view/

This PR adds:

• Support for configuration through YAML scroll-view key. scroll-view: true is a shortcut for

  revealjs:
    scroll-view:
      activate: true
  

  meaning to start the presentation in scroll view by default.

• a menu tool item to toggle scroll view on and off, associated with new R keyboard shortcut

• Adapt our custom footer processing to work in Scroll view mode.

• tests for our Quarto specific support (not for the scroll view feature itself as this is revealjs)

There may still be some other side effect I did not find.

We could also so some still choices by default (like hide chalkboard button by default in scroll mode 🤷 )

I think we may discover a few side effect of this new mode, possibly also because of some Revealjs bug. We'll see

TO-DO

• Document this new feature in Quarto-web

[cderv]

Let's document after merging as discussed.