Enables drag-and-drop behaviour in Shiny apps, by exposing the functionality of the 'SortableJS' <https://sortablejs.github.io/Sortable/> JavaScript library as an 'htmlwidget'. You can use this in Shiny apps and widgets, 'learnr' tutorials as well as R Markdown. In addition, provides a custom 'learnr' question type - 'question_rank()' - that allows ranking questions with drag-and-drop.
