text: Title: Support argument conversion and named arguments with dynamic variable files
  Body: If we have a dynamic variable file like
  ```python
  def get_variables(a: int = 0, b: float = 0.0):
      ...
  ```
  using argument conversion like
  ```
  Variables    example.py    42    3.14
  ```
  and also using named arguments like
  ```
  Variables    example.py    b=${3.14}
  ```
  doesn't work.
  
  This is different to library imports, keywords, and also to extensions like listeners. We have the needed functionality to support argument conversion and named arguments available and hooking it up with variable files is easy. Let's do that!
  