nl: fix zero padding of negative line numbers

[cakebaker]

This PR fixes the zero padding of negative line numbers when using -nrz/--number-format=rz. It also moves the logic for formatting line numbers to its own function.

// before change
$ echo "test" | cargo run nl -v-10 --number-format=rz
000-10  test

// after change
$ echo "test" | cargo run nl -v-10 --number-format=rz
-00010  test

This PR probably conflicts with #5091.

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/rm/rm2 is no longer failing!
Skipping an intermittent issue tests/tail-2/inotify-dir-recreate

[cakebaker]

I just realized when fixing the merge conflict that most of #5091 was not necessary :|

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail-2/inotify-dir-recreate