feat: remove consequent newlines

[logic-finder]

It would be a good thing to add a feature that makes consequent 2 or more newline into one, e.g.

$ cat demo.c
int main(void) {
   int a;



   return a;
}
$ proofread -c --dry-run - < demo.c
> proofread -c --dry-run=full -
=================================================
   1|int main(void) {
   2|   int a;
   3|
   4|-\n
   5|-\n
   6|   return a;
   7|}

Note that the line 4 and 5 have been marked with "-\n", which means these two newlines are redundant.