refactor: share TTY attribute escape building

The bold/underline/italic SGR block was duplicated between the
indexed and true-colour formatters; attrEscapeSequence now builds
it once. Colour rendering stays with each formatter as it
legitimately differs.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: alecthomas

formatters/tty_indexed.go

@@ -181,7 +181,9 @@ var ttyTables = map[int]*ttyTable{
 	},
 }
 
-func entryToEscapeSequence(table *ttyTable, entry chroma.StyleEntry) string {
+// attrEscapeSequence returns the SGR escape codes for entry's bold, underline
+// and italic attributes.
+func attrEscapeSequence(entry chroma.StyleEntry) string {
 	out := ""
 	if entry.Bold == chroma.Yes {
 		out += "\033[1m"
@@ -192,6 +194,11 @@ func entryToEscapeSequence(table *ttyTable, entry chroma.StyleEntry) string {
 	if entry.Italic == chroma.Yes {
 		out += "\033[3m"
 	}
+	return out
+}
+
+func entryToEscapeSequence(table *ttyTable, entry chroma.StyleEntry) string {
+	out := attrEscapeSequence(entry)
 	if entry.Colour.IsSet() {
 		out += table.foreground[findClosest(table, entry.Colour)]
 	}

formatters/tty_truecolour.go

@@ -54,16 +54,7 @@ func trueColourFormatter(w io.Writer, style *chroma.Style, it iter.Seq[chroma.To
 			continue
 		}
 
-		formatting := ""
-		if entry.Bold == chroma.Yes {
-			formatting += "\033[1m"
-		}
-		if entry.Underline == chroma.Yes {
-			formatting += "\033[4m"
-		}
-		if entry.Italic == chroma.Yes {
-			formatting += "\033[3m"
-		}
+		formatting := attrEscapeSequence(entry)
 		if entry.Colour.IsSet() {
 			formatting += fmt.Sprintf("\033[38;2;%d;%d;%dm", entry.Colour.Red(), entry.Colour.Green(), entry.Colour.Blue())
 		}