Template Hierarchy: Extend to include root taxonomy archives

[ockham]

WIP. Based on #7271. More details to follow.

Nomenclature

For a given taxonomy projects that has terms such as film or tv:

1. The root taxonomy route is /projects.
2. Non-root taxonomy routes are any routes that include not only the taxonomy but also any of the taxonomy's terms, e.g. /projects/film or /projects/tv.

What

WordPress currently only supports non-root taxonomy routes. They are handled by this part of the template hierarchy:

(With an eventual fallback to archive.html and finally index.html, not depicted above.)

In #7271, I'm proposing a new flag to register_taxonomy() that will allow WordPress to support root taxonomy routes for a given taxonomy; however, that PR doesn't make any changes to the template hierarchy -- instead falling back to the generic archive template (archive.html).

This PR proposes to extend the template hierarchy to include a dedicated template for root taxonomy routes.

How

As discussed in #13816, there are two major options for what a root taxonomy archive should display:

1. A list of all the taxonomy’s terms (i.e. /projects shows a list of items called TV, Films, etc, but not the individual posts that have those terms assigned).
2. A list of all posts that have any of the taxonomy’s terms assigned (i.e. /projects shows a list of all posts that have either tv or film or any other term from the projects taxonomy assigned).

While 1. is somewhat more consistent with the existing (non-root) taxonomy templates, a majority of people in #13816 seem to have been in favor of 2. My own use case would be a template that can be shared by both non-root and root taxonomy routes.

I thus believe that we should accommodate for both options.

AFAICS, this is best done by introducing two new templates for the root taxonomy archives that take precedence, and eventually fall back to the (non-root) taxonomy templates, like so:

Trac ticket:

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