Problem/Motivation
The current approach on #3050384: Provide a starterkit theme in core only support single starterkit theme to be used as the starting point for generated themes. However, it could be useful to have other themes be available as the starting point.
Proposed resolution
Allow themes to indicate if they can be used as a starting point on the starterkit theme generator. We probably should also provide command that allows finding themes that can be used a starterkit theme.