Grainfmt: Experiment with print_comment_range refactor

[phated]

The print_comment_range function has a grabbag of options. I believe that by adding the comments and a "comment list" type to the Doc.t, we can drop group_breaker and simply the comment logic that happens inside fmt.re. Some of my initial work happened at #2006 but we're deferring further cleanup to land #1976 sooner.