Add support for creating patterns for templates, using template types

[richtabor]

I'd like to be able to create patterns that can be used as templates for use in this replace flow for example. That way creators can build designs for index, single, page, and other templates, and I can swap out my theme's template with one of these.

TT4 does this currently by creating patterns with template types, like this one here. I'd expect to be able to create patterns on the directory in the same fashion, likely using a similar mechanism as block types (#609).

To start, core could ship this to sites much like we do the core header and footer alternatives, but eventually creators should be able to make patterns with template types and as a user I should be able to find those in the editor flows.