Add a `pre_set_object_terms` action

[davidbaumwald]

Working with some custom terms & taxonomies, and it looks like there's no way to hook in to wp_set_object_terms _before_ the terms are updated.

In the end, the $old_tt_ids is passed to the set_object_terms action(added in r12261). But, these are the term_taxonomy_ids and a a bit harder to work with than the plain terms. Also, these are provided _after_ the terms are updated. So, there's no way to short-circuit the updating.

However, I think it would be useful to have a pre_ action that can work on the current terms before commiting the changes.

[sagarsavani7979]

This commit adds a new action hook, 'pre_set_object_terms', to the wp_set_object_terms function. This hook fires before an object's terms are set, allowing developers to perform operations or modifications on the terms array before the changes are committed.