Possibility of "select all"

Hi,

With just some quick tests, I could make it work with a fancyTree extension (https://github.com/mar10/fancytree/wiki/ExtMulti)

• Use all bundle in libraries.yml : //cdn.jsdelivr.net/npm/jquery.fancytree@2.37.0/dist/jquery.fancytree-all.min.js
• In fancyTree options, allow keyboard and add settings :

  keyboard: true,
  multi: {
     mode: "sameParent"  // Restrict range selection behavior
  },
  extensions: ['multi'],
  

I can make a patch if you want, but I need help to test side effects (eg: why keyboard is set to false ?)

Thanks