In `wp_dropdown_categories()`, allow the term field used to populate option valuesto be specified.

The new 'value_field' parameter makes it possible to set term slugs (or some
other term property) as the 'value' attribute of the option elements generated
by `wp_dropdown_categories()`. This additional flexibility reduces the effort
required to translate term_id to other term fields when processing form
submissions that include values from taxonomy dropdowns. See #30865 for a
use case.

Props collinsinternet.
Fixes #30306.
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