[6.x] Various drag & drop fixes

[duncanmcclean]

This pull request fixes various drag & drop issues:

• Fixes Bard sets not being draggable
  • This was happening because our workaround for a Firefox bug ([6.x] Fix Firefox not receiving text selection in nested Bards #14021) set draggable="false" on the node-view-wrapper element, preventing the native dragstart events from firing, which TipTap needs to handle drag and drop.
  • This PR fixes it by adding draggable="true" directly to the drag handle element, allowing the native dragstart event to fire from the handle and bubble up to TipTap's event handler, while the wrapper remains draggable="false" to preserve the Firefox text selection fix.
  • Fixes Can't move set unless its focused within Bard #14191
• Fixes relationship items not being draggable when going from "0 -> some" items
  • This was happening because makeSortable() is only run when the ref="items" element exists, but it's wrapped inside a v-if which evaluates to false when there aren't any items.
  • I've fixed it by calling makeSortable() when items are added to an empty relationship field.
  • Fixes Can not drag/move linked items up/down #14124