gh-125355: Rewrite parse_intermixed_args() in argparse (GH-125356)

* The parser no longer changes temporarily during parsing.
* Default values are not processed twice.
* Required mutually exclusive groups containing positional arguments are
  now supported.
* The missing arguments report now includes the names of all required
  optional and positional arguments.
* Unknown options can be intermixed with positional arguments in
  parse_known_intermixed_args().