Fix: Site Editor Template part is missing an icon in the sidebar

[dhananjaykuber]

Fixes #68649

What?

Fixes the missing icon display for Template Parts in the Site Editor's document settings sidebar by implementing proper icon mapping for template part areas.

Why?

After #66459, the document settings sidebar stopped displaying icons for Template Parts because the icon value returned from default_template_part_areas was a string that wasn't properly mapped to a WordPress icon component. This PR restores the icon display functionality.

Testing Instructions

1. Open the Site Editor
2. Navigate to a Header template part (or create one if none exists)
3. Observe that the document settings sidebar now displays the correct icon
4. Repeat the process with Footer and Sidebar template parts to verify icons display correctly

Screenshots or screencast

[github-actions]

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Co-authored-by: dhananjaykuber <dhananjaykuber@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>
Co-authored-by: im3dabasia <im3dabasia1@git.wordpress.org>

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[Mamaduka]

Thanks, @dhananjaykuber!