Privacy: Introduce a JSON file into the personal data export.

The produced ZIP archive will now include an `export.json` file along with the current `index.html`.

Props xkon.
Fixes #49029. See #46424.
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git-svn-id: https://core.svn.wordpress.org/trunk@46946 1a063a9b-81f0-0310-95a4-ce76da25c4cd