Ensure block editor preloading middleware JSON is correctly escaped

[sirreal]

Fixes #62797 by correctly escaping JSON included inside JavaScript in a script tag.

wp_add_inline_script will correctly escape closing script tags </script> which prevents script contents from breaking out of the script, it does not handle cases where some content may enter the HTML script data double escaped state and prevent the real script closing tag from closing the script tag as expected, leading to a broken document where the script tag remains open.

This can be tested by applying the patch and following the reproduction steps in #62797.

Trac ticket: https://core.trac.wordpress.org/ticket/62797

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