Add `before_delete_term` action

[cmmarslender]

Sometimes, the need arises to hook in to wp_delete_term, and take some action before the terms are actually deleted. Currently, there are only actions that fire after changes are already made in the database (term gone, posts already updated, etc). Would be really helpful to be able to hook into this process before any changes are made.

Patch attached with proposed addition.

[lancewillett]

Patch looks good to me.

[boonebgorges]

'pre_' is much a much more common convention for this sort of hook in WP. We do have 'before_delete_post', but I think that's just a very old hook.

[boonebgorges]

In 30088:

Introduce 'pre_delete_term' hook.

Props cmmarslender.
Fixes #30127.