Show appender only when item has submenu

[tellthemachines]

Description

Closes #18104.

Navigation menu items: Block appender should only be visible when the item already has a submenu.
If the item has no submenu, show a toolbar button that reveals the block appender on click.

Would love some a11y feedback on whether clicking the toolbar button should merely show/hide the appender, or if we should transfer focus to the appender, or announce its presence in any other way.
Currently clicking only shows/hides the appender so screen reader users won't get any feedback on what is happening.

How has this been tested?

Tested in browser:

1. Enable navigation block in experiments
2. Add a navigation block with some nav item blocks to a post
3. Check that toolbar button is rendered for nav items that have no children, and appender button is not rendered.
4. Check that toolbar button is not rendered for nav items that do have children, and appender button is rendered.

Screenshots

Scenario 1:

Scenario 2:

Types of changes

New feature (non-breaking change which adds functionality)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.

[tellthemachines]

Actually, now that #16708 has been merged, it makes more sense for the toolbar button to immediately insert a block. Will update.

[draganescu]

I have two minor comments, but otherwise this is good to go.