Print CSS rule to fix potential visual issues with auto-sizes

[felixarntz]

This PR implements what's described in https://core.trac.wordpress.org/ticket/62413#comment:36. It also introduces a filter to opt out of sizes=auto.

With this fix:

• Images that have sizes=auto (by default all lazy-loaded images in WordPress) and are affected by a CSS rule that sets their width to either auto or fit-content will no longer appear in a smaller size than intended.

Trac ticket: https://core.trac.wordpress.org/ticket/62413

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Props flixos90, joemcgill, apermo, mukesh27.

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[joemcgill]

This looks right. What would you think about bailing early if auto-sizes has been disabled via a filter similar to what is being proposed in #7812?

[felixarntz]

Makes sense to me. Let's either add this here or in #7812, depending on which of the two gets committed first. We can coordinate that once we're confident about the changes.

[felixarntz]

Added in 3b2a48e

[github-actions]

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[apermo]

LGTM, optional suggestion on adding return type.

[joemcgill]

This looks good to me

[felixarntz]

Committed in https://core.trac.wordpress.org/changeset/59415