New label for post types and taxonomies: "template_name"

[tw2113]

From WP Trac #60881

Add the label template_name to post types and taxonomies for use by the site editor to display on the templates/add new template screens to allow extenders to customize the display.

For post types the template name applies to templates for a singular pages with the default value "Single item: [singular name]".

For taxonomies the template name applies to templates for term archives with the default value "[singular name] Archives".

Requires WP 6.6.x to apply.