T2 Add before/after Supports

Adds Add before and Add after menu controls to inner block contexts where only single block type is allowed. The default Add before/Add after from core requires that core/paragraph is allowed as inner block.

link Example

block.json

{
	"name": "t2/accordion",
	"supports": {
		"t2AddBeforeAfter": "t2/accordion-item"
	}
    …
}