Introduce some new template functions for archive titles and descriptions:

 * `get_the_archive_title()` and `the_archive_title()` for returning/displaying the title of the current term, date, post type, post format, or author archive.
 * `get_the_archive_description()` and `the_archive_description()` for returning/displaying the description associated with the current term archive.

Fixes #21995
Props obenland, DrewAPicture

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