ISBAT add an accordion item using a placeholder when interacting with the parent block

**Is this enhancement related to a problem? Please describe.**
After baking in a new UX pattern with omnipresent placeholders in the Menu/Menu Item block, we should consider adding this pattern to the Accordion block.

**Describe the solution you'd like**
The Accordion block's "Add Accordion Item" appender should be replaced with a UX mechanism similar to the new Menu/Menu Item blocks, where a placeholder Accordion Item block appears when interacting with the block.

**Current:**

**Proposed:**