Indicate submenu item level in admin nav menu editor and customizer nav menu editor

[Rcreators]

Trac ticket: https://core.trac.wordpress.org/ticket/60673

---

This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core Committers: Use this line as a base for the props when committing in SVN:

Props rcreators, joedolson, afercia.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[github-actions]

Test using WordPress Playground

The changes in this pull request can previewed and tested using a WordPress Playground instance.

WordPress Playground is an experimental project that creates a full WordPress instance entirely within the browser.

Some things to be aware of

• The Plugin and Theme Directories cannot be accessed within Playground.
• All changes will be lost when closing a tab with a Playground instance.
• All changes will be lost when refreshing the page.
• A fresh instance is created each time the link below is clicked.
• Every time this pull request is updated, a new ZIP file containing all changes is created. If changes are not reflected in the Playground instance,
  it's possible that the most recent build failed, or has not completed. Check the list of workflow runs to be sure.

For more details about these limitations and more, check out the Limitations page in the WordPress Playground documentation.

Test this pull request with WordPress Playground.

[Rcreators]

@afercia All changes have been amended.

[afercia]

Please see inline comments

[Rcreators]

Please see inline comments

All the requested changes are pushed.

[afercia]

Thanks for addressing the coding standards points I raised @Rcreators
I understand part of this PR is code copied from already existing code there's some more occurrences of missing spaces between parehthesis and single quote (' or ') that would be nice to fix if possible. Thanks!
Other than that, I would leave functional and final review to @joedolson

[joedolson]

In r58306