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.

src/printer.rs

@@ -738,10 +738,17 @@ impl Printer for InteractivePrinter<'_> {
 
         // Line decorations.
         if self.panel_width > 0 {
+            let display_line_number =
+                if self.config.number_nonblank && line.trim_end_matches(['\r', '\n']).is_empty() {
+                    0
+                } else {
+                    line_number
+                };
+
             let decorations = self
                 .decorations
                 .iter()
-                .map(|d| d.generate(line_number, false, self));
+                .map(|d| d.generate(display_line_number, false, self));
 
             for deco in decorations {
                 write!(handle, "{} ", deco.text)?;