text: Title: Support setting system prompt
  Body: Support setting the system message.
  
  ```python
  from magentic import prompt
  
  
  @prompt(
      'Add more "dude"ness to: {phrase}',
      system="You are the coolest dude ever.",
  )
  def dudeify(phrase: str) -> str:
      ...
  ```
  
  - `system` should support templating in the same way that `template` does.
  - (future) `prompt` should output a warning that `system` is being ignored when it is passed in combination with a completions model (if/when support for those is added).