Convert Outside-in's Tray to a core element

With the addition of the Tray modal type from #2753941: [Experimental] Create Outside In module MVP to provide block configuration in Off-Canvas tray and expand site edit mode and the progression of #2739079: Reduce visual clutter of Place Block module the functionality of the Tray is becoming more clearly an item more modules will want to make use of [ie: paragraphs.

also then UX and a11y issues with the current Tray JS can be handled more directly as a core element.