Newspack Listings: Add option to set a default template

[laurelfulford]

Is your feature request related to a problem? Please describe.
Right now, the Newspack Listings will automatically use the One Column Wide template by default. This isn't always the best choice for listings that are all text (and don't use columns blocks).

Describe the solution you'd like
To be able to set a default template to be used for all new Listings in the Customizer, similar to post and pages.

[estereofonico]

This is a greta idea, it's possible to launch this option? We'd love to create more templates for our content by categories, for example, make the editor select the template called xyz Category as a custom predesign. This could be useful to include beautifully other design options with no effort from the user.