feat/570: Enhancements to menu item classes

[Sidsector9]

Description of the Change

Closes #570

Adds the following:

• Support for custom menu item HTML classes.
• menu-item-has-children HTML class for menu item with children.

How to test the Change

1. Perform the basic setup.
2. Add children to a menu item.
3. Add a custom CSS class to a menu item.
4. Verify on the frontend.

Changelog Entry

Added - Support for custom menu item CSS classes. Added - menu-item-has-children CSS class for menu item with children.

Credits

Props @burhandodhy @Sidsector9

Checklist:

• [ ] I agree to follow this project's Code of Conduct.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my change.
• [ ] All new and existing tests pass.