keep-sorted could handle trailing commas better

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

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

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

```java
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

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

would become

```java
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

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

would become

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