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

@@ -480,6 +480,9 @@ helpVariables(unsigned short int pager)
 		  "    border style (number)\n");
 	HELP0("  columns\n"
 		  "    target width for the wrapped format\n");
+	HELPN("  csv_fieldsep\n"
+		  "    field separator for CSV output format (default \"%c\")\n",
+		  DEFAULT_CSV_FIELD_SEP);
 	HELP0("  display_false\n"
 		  "    set the string to be printed in place of a boolean 'false'\n");
 	HELP0("  display_true\n"