Add dynamic social share images for plugin pages (Meta #5926)

[mathetos]

Fixes https://meta.trac.wordpress.org/ticket/5926

Plugin pages now get a proper preview image when someone shares a link on social media. You get the title, description, icon, and a few key stats in one clean 1200x630 image.

This follows what Dion asked for on #5926: build it in PHP, keep it simple, and serve dynamic images the same way we already do with geopattern icons so the CDN can cache them. Jetpack stays installed, and on plugin pages we let it step aside for Open Graph and Twitter tags so social platforms use our image.

The layout uses a small set of constants and a straightforward stats grid that holds up across different plugin titles and descriptions. The change stays focused on the plugin directory and theme meta tags, and it passes PHPUnit and PHPCS.

Sample output

Test plan

• PHPCS passes on new share image files
• Plugin Directory PHPUnit suite passes (127 tests)
• Visit /share-image/{slug}.jpg on a plugin page and confirm JPEG renders
• View page source on a plugin page and confirm og:image points at the share image URL
• Confirm Jetpack does not output duplicate og:image tags on plugin pages

[github-actions]

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