text: Title: Enhance programmatic API to create resource files
  Body: The main motivation is making it easier to create resource files so that they can be converted to JSON. JSON support it self is #3902. `ResourceFile` already has `to_json` and `from_json` methods, but two additional things are needed to make it's usage more convenient:
  
  - It needs to be exposed via `robot.running`.
  - It needs `from_file_system`, `from_string` and `from_model` class methods that also `TestSuite` has.
  
  After these enhancement this is possible:
  ```python
  from robot.running import ResourceFile
  
  resource = ResourceFile.from_file_system('example.resource')
  resource.to_json('example.rsrc')
  ```