Primary flag for term archives

This release introduces the ?primary=true query var. Appending that query string to the URL of a taxonomy archive page will limit posts to ones that feature the current term as the primary term.

Initial pre-release

This release provides support for primary terms on any post type using the Classic editor. It also will move the primary term to the beginning of the post terms array. Dynamically adding terms to the primary select is currently only functional for Categories. Support for Gutenberg is also in progress.