text: Title: Add `__repr__()` method to NormalizedDict
  Body: I see the problem, that with the RobotDebug REPL i am updating right now, the NormalizedDict are just shown like this:
  
  `${SUITE_METADATA} = <robot.utils.normalizing.NormalizedDict object at 0x1048063e0>`
  
  Could we do it like this?
  ```python
  def __repr__(self):
      return '%s(%s)' % (self.__class__.__name__, str(self))
  ```
  or:
  ```python
  def __repr__(self):
      return f'{self.__class__.__name__}({self.__str__()})'
  ```
  Then it would be :
  `${SUITE_METADATA} = NormalizedDict({'Hello': 'World', 'Test': '123'})`
  
  We could also write it like this?
  `robot.utils.NormalizedDict({'Hello': 'World', 'Test': '123'})`
  
  But i think the former version would be good enough 
  
  What do you think?
  