Ensure feature parity between Navigation Link and Navigation Submenu blocks.

[getdave]

What problem does this address?

The core/navigation-link and core/navigation-submenu blocks are inconsistent with each other, despite the need for feature parity between them.

For example top level submenu items should have feature parity with top level link items.

However, as they do not share the same codebase a number of inconsistencies are in evidence which causes headaches for users, especially around styling. UI consistency would also help avoid confusion when editing menus.

What is your proposed solution?

Initially I propose we abstract and share key functionality between the blocks. We can do this by sharing code. Is their any precedent for this in the block-library?

In the longer term it might be possible to merge the blocks and remove the need for a separate submenu block.