add separator between args in help message

[vanyakosmos]

maybe via Meta class with parameters (django-style)

class Args:
  a: int
  b = 'foo'
  c = 'bar'

  class Meta:
    groups = [
      ['a', 'b'], ['c'],
    ]
    # or
    order = 1  # or -1, or list of names
    separator = ['c']  # place separator above `c` arg

or as list in parse_args:

parse_args([Args, AnotherArgs, ...])

or pytest style: anonymous OptionGroup + list of groups