Resolves #73647

Alternative to #73648

Introduce a toolbar-based [+] inserter for the Navigation block, replacing the in-canvas inserter.

Before	After

Why

The inline inserter inside the Navigation block often leads to:

• Layout shifts as it appears/disappears within the menu
• Ambiguity about which part of the menu it relates to
• Extra complexity in an already nested editing experience

A toolbar-based inserter provides:

• A stable, predictable insertion point
• Clearer association with the Navigation block itself
• A cleaner canvas with fewer shifting elements

How

• Introduce re-usable ToolbarBlockAppender component, similar to ButtonBlockAppender:
  • Implements new block inserter using [ + ] icon
  • Reused the standard Inserter component with toggleProps for consistency
  • Creates a new toolbar group at the end of the block toolbar using BlockToolbarLastItem
  • Defaults to inserting a paragraph directly, but can also be used to insert any block via defaultBlock prop
• Replace the existing “Add page” control with the new component
• Styled the button to match the standard block inserter (black square with rounded corners, white plus icon)
• Removed the in-canvas inserter for the Navigation block

Questions/thoughts

• Should open the dropdown to select a block by default, or always directly insert a block?
  

• Should check if block allows inner blocks before adding itself to the toolbar?

• Should pull the default inner block from the block where this is used, thus not requiring passing the default block to the component?

Testing Instructions

• In navigation block, use the block inserter to add new pages.
• Test template editor and focused template part editor

Testing Instructions for Keyboard

Screenshots or screencast