Have psql's "\? variables" show csv_fieldsep

Accidental omission in commit aa2ba50c2c.  There are too many lists of
these variables ...

Discussion: https://postgr.es/m/202511031738.eqaeaedpx5cr@alvherre.pgsql

src/bin/psql/help.c

@@ -441,6 +441,9 @@ helpVariables(unsigned short int pager)
 					  "    border style (number)\n"));
 	fprintf(output, _("  columns\n"
 					  "    target width for the wrapped format\n"));
+	fprintf(output, _("  csv_fieldsep\n"
+					  "    field separator for CSV output format (default \"%c\")\n"),
+			DEFAULT_CSV_FIELD_SEP);
 	fprintf(output, _("  expanded (or x)\n"
 					  "    expanded output [on, off, auto]\n"));
 	fprintf(output, _("  fieldsep\n"