feat: implement -b / --number-nonblank for cat -b compatibility

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.

doc/short-help.txt

@@ -33,6 +33,8 @@ Options:
           Truncate all lines longer than screen width. Alias for '--wrap=never'.
   -n, --number
           Show line numbers (alias for '--style=numbers').
+  -b, --number-nonblank
+          Show line numbers for non-blank lines only (alias for '--style=numbers').
       --color <when>
           When to use colors (*auto*, never, always).
       --italic-text <when>