text: Title: Dynamic API: Support positional-only arguments
  Body: RF 4.0 added support for Python's positional-only arguments (#3695). It isn't that important feature, but sometimes it comes handy. For consistency reasons also dynamic libraries should support it.
  
  Implementing this enhancement shouldn't be too complicated. Our `ArgumentSpec` already supports positional-arguments and that's what is used during execution. The only needed change ought to be enhancing the code that parses argument information returned by dynamic libraries. Well, even before that we needed to agree on the syntax but `['posonly', '/', 'normal']` that matches the syntax used by Python is a pretty obvious candidate. It's also consistent how the dynamic API supports named-only arguments like `['normal', '*', 'namedonly']`.