CreateTemplatePartModal: avoid identity warning in useSelect

[jsnajdr]

This patch should fix the useSelect warning that @mirka reports in #67702 (comment). It happens because the defaultTemplatePartAreas select returns a different value on every call, even though the inputs are exactly the same. That's because:

• the [] default empty array is a different instance every time
• the areas.map call always returns a different array, even though the input and the contents of the output are always the same

I'm fixing this by extracting the [] defaulting and the .map call outside the useSelect hook. The defaultTemplatePartAreas value is processed by another .map call, so I'm merging both into one. Computing the icon value just-in-time when rendering.

[github-actions]

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Co-authored-by: tyxla <tyxla@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>

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[tyxla]

Fixes the warning in the new template part / duplicate template part modal 👍

Thanks @jsnajdr 🚀

[github-actions]

Flaky tests detected in 08773a3.
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/12254426886
📝 Reported issues:

• [Flaky Test] submenu click on the arrow interactions #61884 in /test/e2e/specs/editor/blocks/navigation-frontend-interactivity.spec.js

[Mamaduka]

Thanks, @jsnajdr!

[gigitux]

Thanks! 🙇