Ensure that `wp_list_categories()` supports comma-separated lists for 'exclude' and 'exclude_tree'.

[34696] introduced a regression whereby comma-separated values for 'exclude'
and 'exclude_tree' would be handled improperly when merging the two parameters,
resulting in category IDs being incorrectly dropped from the combined array.

Props gblsm, hnle.
Fixes #35156.
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