[PP-1] Consider using SortableJS spill plugin when dragging sections into layout

Overview

[PP-1] on #3512455: Implement new drag-and-drop solution that removes the need to drop items into the preview iFrame

If you begin dragging a section into Experience builder from the sidebar, there is no way to stop this halfway through, meaning you have to place the section then remove it. It's not the worst UX but we can make it better with the OnSpill plugin. This plugin is already included in our build of Sortable so nothing new needs to be added.

Proposed resolution

Start dragging a section
Drag it outside of the Experience builder layout (like over the sidebar)
Stop dragging
Note that the section was still added to the Layout.

User interface changes

Use the OnSpill plugin (Most likely with the revertOnSpill configuration option so that the section is not added to Experience builder if the user drags the section outside of the experience builder layout.