Low priority.

I want a way to list templates that one can call.

I'm thinking this syntax:
  gn ls out/Default --type=template

This would need to be a special-case since the argument won't be a label or label pattern, and we would also want to list the file the template was declared in.

May want to consider some type of annotation for "internal" templates that shouldn't be listed here if the list is too long.