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.

CHANGELOG.md

@@ -8,6 +8,7 @@
 
 ## Features
 
+- Add `-b` / `--number-nonblank` flag to only number non-blank lines, for `cat -b` compatibility. Closes #3856, see #3857 (@MeGaurav4)
 - Add a `--sanitize=<auto|always|never>` flag for safe display of untrusted input. It implies `--strip-ansi` at the same value and additionally substitutes terminal-active control bytes (cursor moves, charset switches, beep, etc.) and Unicode bidi / zero-width formatting characters with the Unicode replacement character (U+FFFD). Mitigates Trojan-Source-style spoofing (CVE-2021-42574). See #3729 (@curious-rabbit)
 - Map justfile, Justfile, .justfile, and *.justfile to Makefile syntax highlighting, see #3623 (@zachvalenta)
 - Preserve `--diff` change markers and snip separators when `--plain` is set. Closes #3630, see #3643 (@mvanhorn)