fix #64581 - Display serialized option values instead of placeholder text

[Presskopp]

Fixes https://core.trac.wordpress.org/ticket/64581

This replaces the hard-coded "SERIALIZED DATA" placeholder in
wp-admin/options.php with a read-only display of the actual
unserialized option value.

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[github-actions]

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[huzaifaalmesbah]

Could we place this JavaScript in an external file and load it via wp_enqueue_script() for better maintainability?

[Presskopp]

I think we should first wait for a dev feedback on the feasibility of this overall concept. Then we can talk about the specific details.

[anatolinicolae]

What about a native approach instead? Like relying on the <details> tag that basically does the same thing as this JS. I'll give it a try locally later anyway 👍