Navigation: Share submenu icon code

[scruffian]

What?

Share the submenu icon code between the navigation link and submenu block.

Why?

We should share duplicated code to help with maintenance.

How?

Move the shared code to the navigation-link/shared directory.

Testing Instructions

1. Add a navigation block with submenus
2. Open the frontend of the site
3. Confirm that there are icons by the submenus on the navigation block.

[github-actions]

Flaky tests detected in d47968e.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/22403903731
📝 Reported issues:

• [Flaky Test] should apply Heading block styles to all Heading blocks #49942 in /test/e2e/specs/site-editor/push-to-global-styles.spec.js