Post featured image: add example of the block

[MaggieCabrera]

What?

Add an example to the core/post-featured-image block.

Part of #30029

Why?

Because it was showing empty before

How?

By defining a viewport width size for the example

Testing Instructions

Create a post and add a featured image to it.
Open the block inserter and search for the featured image block, hover over it and you'll see the example. It should show the current post's featured image.

Testing Instructions for Keyboard

Screenshots or screencast

[github-actions]

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Co-authored-by: MaggieCabrera <onemaggie@git.wordpress.org>
Co-authored-by: jameskoster <jameskoster@git.wordpress.org>

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[MaggieCabrera]

This one is showing empty on global syles:

[github-actions]

Flaky tests detected in bb18747.
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/9796999474
📝 Reported issues:

• [Flaky Test] should open the "Manage fonts" modal when clicking the "Manage fonts" button #63074 in /test/e2e/specs/site-editor/font-library.spec.js

[MaggieCabrera]

This PR needs #63177 to land for the GS previews to work:

GS	Inserter

[jameskoster]

The preview is a bit lacklustre when there's no context, but that's a separate issue.