Add an alias to target all slides format in `quarto.doc.isFormat()`

[cderv]

Bug description

It could be useful to add one alias for all slides output in our the format detection utility function (https://quarto.org/docs/extensions/lua.html#format-detection) so that it is easy to target slides in a Lua filter only for slides.

Example: Removing Pause syntax

Para = function(e)
  if (not quarto.doc.isFormat("revealjs") or not quarto.doc.isFormat("beamer") or not quarto.doc.isFormat("pptx")) then
    if (e == pandoc.Para '. . .') then
      return {}
    end
  end
  return nil
end