Improve compatibility of styling picture elements

[devansh016]

Summary

Fixes #1300

Relevant technical choices

Added following style to picture tag and updated the test for the same.

style="display: contents;"

The wp:gallery style was not getting applying due to the img getting inside the picture tag.

[github-actions]

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Co-authored-by: devansh016 <devansh2002@git.wordpress.org>
Co-authored-by: mukeshpanchal27 <mukesh27@git.wordpress.org>
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