Unify implementations of converting WP menu items to Nav Link blocks between nav editor and block

[getdave]

What problem does this address?

Currently we have two locations within the codebase that convert WP Nav Menu items into core/navigation-link blocks:

1. Nav block - when creating a block from existing menu items.
2. Nav editor - when rendering a menu within the editor.

These do (almost!) exactly the same thing but differ in their implementation.

What is your proposed solution?

I am proposing that we unify under a single well-tested implementation. Once the implementation is in place, we can (for now) then duplicate it between block and editor.

In the longer term, we can then remove the duplication by (hopefully) placing the conversion code into the proposed @wordpress/menus package.

Note I would do this in the PR that proposes the package, but it would cause the PR to become overly complex and large. I'd prefer to refactor in isolation from the package PR and then rebase that PR when we are confident we have a stable implementation across both codebases.

[getdave]

Closed by #31606