Allow for vertical tabset panel

Discussed in https://github.com/quarto-dev/quarto-cli/discussions/3963

^{Originally posted by kbvernon January 13, 2023}
So I don't come off as demanding, let me start off by saying Quarto is awesome. I'm very grateful for all the work you all put in to developing it. OK, so now for the FR.

tabby.js allows for vertical tabsets, you just have to set the display to grid. Would be nice if this could be implemented in Quarto. Here's some css based on the suggestion from the author of tabby:

@media (min-width: 30em) {

  .panel-tabset {
     display: grid;
     grid-gap: 2em;
     grid-template-columns: 25% 75%;
  }
  
  .panel-tabset-tabby {
     border-bottom: none !important;
     border-right: 1px solid #bbb; 
  }
  
  .panel-tabset [role=tab][aria-selected=true] {
     background-color: transparent;
     border: 1px solid #bbb !important;
  }
  
  h3 {
     margin-top: 0;
  }
  
}

Maybe this could be its own css class for use in Quarto, something like .panel-tabset-vertical? Would also be useful being able to set the relative column widths like:

::: {.panel-tabset-vertical columns="25% 75%"}
### First Tab

Thanks!

* Original discussion about this FR is over at RStudio Community.

Start with discussion #3963 and the linked RStudio Community thread, then inspect the existing panel-tabset implementation and its Tabby.js/CSS behavior. The proposed scope is a vertical tabset option, potentially using a panel-tabset-vertical class and configurable columns such as 25% 75%; done should include working vertical panels without breaking the existing layout.