Return the taxonomy object in register_taxonomy()

[SergeyBiryukov]

Background:

• register_post_type() returns the registered post type object on success, a WP_Error object on error.
• register_taxonomy() returns nothing on success, a WP_Error object on error.

From a consistency point of view, I think it would make sense for register_taxonomy() to return the registered taxonomy object as well.

Some history, for context:

• [5525] introduced register_taxonomy().
• [11998] introduced register_post_type().
• [12597] introduced a return value for register_post_type(), the $args array at the time.
• [37890] introduced WP_Post_Type object, and changed register_post_type() return value to that.
• [38747] introduced WP_Taxonomy object, but not as a register_taxonomy() return value, as it never had one.

[SergeyBiryukov]

In 47186:

Taxonomy: In register_taxonomy(), return the registered taxonomy object on success, for consistency with register_post_type().

Props krynes, SergeyBiryukov.
Fixes #48558.

[SergeyBiryukov]

In 47187:

Docs: Add a @since note for [47186].

See #48558.