Add `saved_term` actions that fire on create and update

[dlh]

The save_post and save_post_{$post_type} actions fire both when a post is created or updated. Terms have separate created_* and edited_* hooks, but not a shared hook like posts do.

So, if a developer wants to hook into both the create and update events, two add_action()s are required for terms, but only one is required for posts.

This patch proposes adding new saved_term and saved_{$taxonomy} hooks to wp_insert_term() and wp_update_term() to bring the same convenience to terms currently available for posts.

[davidbaumwald]

@SergeyBiryukov Latest patch still applies cleanly. Is this on your list to review for 5.5?

[SergeyBiryukov]

In 48314:

Taxonomy: Introduce saved_term and saved_{$taxonomy} actions to complement created_term and created_{$taxonomy}.

This allows for hooking into both the create and update events with a single callback, in the same way that is already possible for posts via the save_post and save_post_{$post->post_type} actions.

Props dlh.
Fixes #48416.