read-print: Correctly handle comments that follow a list head.

Fixes <https://issues.guix.gnu.org/61013>.
Reported by Maxim Cournoyer <maxim.cournoyer@gmail.com>.

* guix/read-print.scm (pretty-print-with-comments)[starts-with-line-comment?]:
New procedure.
Use it when printing a list.
* tests/read-print.scm: Add two tests.

tests/read-print.scm

@@ -210,6 +210,15 @@ mnopqrstuvwxyz.\")"
  \"abcdefghijklmnopqrstuvwxyz\")"
                    #:max-width 33)
 
+(test-pretty-print "\
+(list ;margin comment
+      a b c)")
+
+(test-pretty-print "\
+(list
+ ;; This is a line comment immediately following the list head.
+ #:test-flags #~(list \"-m\" \"not external and not samples\"))")
+
 (test-pretty-print "\
 (modify-phases %standard-phases
   (replace 'build