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.

git-svn-id: https://develop.svn.wordpress.org/trunk@31006 602fd350-edb4-49c9-b593-d223f7449a82