chore(Tiered List): Refactor tiered list macro to extract duplicated code blocks

[Nayab-Gauhar]

Extract duplicated code blocks into private helper macros to improve maintainability.

Created two private helper macros:

• _vf_tiered_list_item_separator(): Renders the HR separator between list items
• _vf_tiered_list_item(): Renders list item content with parameterized column classes

This eliminates code duplication while maintaining the exact same functionality.

Fixes #5530

Done

• Extracted HR separator block into _vf_tiered_list_item_separator() macro
• Extracted list item content block into _vf_tiered_list_item() macro with parameterized column classes
• Replaced duplicated code blocks with macro calls
• Verified that functionality remains unchanged

QA

• The changes are purely structural/refactoring - no visual or functional changes
• All existing tiered list examples should continue to work exactly as before
• The macro API remains unchanged

Check if PR is ready for release

• [ ] PR should have one of the following labels: Maintenance 🔨 (suggested for this refactoring)
• [ ] Vanilla version in package.json should be updated (this is a maintenance/refactoring change, so a bugfix release x.x.X would be appropriate)
• [ ] No changes to component class names or macro APIs, so no updates needed to what's new page

[webteam-app]

Nayab-Gauhar is not a collaborator of the repo