Adds a `-b` / `--number-nonblank` flag that numbers only non-blank
lines. Blank lines (those containing only the line terminator) are
printed without a number and don't advance the counter, matching
`cat -b` behaviour.
- `-b` implies `--style=numbers` (same as `-n`), but only counts
non-blank lines
- `-b` takes precedence over `-n`: both `-bn` and `-nb` behave like `-b`
- `-bp` disables numbering (plain mode wins)
- Works with files, stdin, and piped output (like `-n`)
Closes#3856.