keep-sorted could handle trailing commas better

[JeffFaer]

There's some logic in keep-sorted to try and gracefully handle lists that don't have a final trailing comma like

ImmutableList.of(
  // keep-sorted start
  1,
  3,
  2
  // keep-sorted end
);

would become the following (commas added and removed as necessary)

ImmutableList.of(
  // keep-sorted start
  1,
  2,
  3
  // keep-sorted end
);

https://github.com/google/keep-sorted/blob/62dd1440b1c8456b372bff09161ea9d847247671/keepsorted/block.go#L338-L340

There's a couple edge cases here that aren't handled very well right now

1. Trailing comments after a comma

   This prevents the special case from being triggered

   ImmutableList.of(
     // keep-sorted start
     1,
     3, // three
     2
     // keep-sorted end
   );

   would become

   ImmutableList.of(
     // keep-sorted start
     1,
     2
     3, // three
     // keep-sorted end
   );

2. Trailing comment after the last line

   The logic still triggers, but we add a comma to the comment

     ImmutableList.of(
       // keep-sorted start
       1,
       3, 
       2 // two
       // keep-sorted end
     );

   would become

     ImmutableList.of(
       // keep-sorted start
       1,
       2 // two,
       3
       // keep-sorted end
     );