text: Title: DAGMapping should ignore non-notebook tasks
  Body: DAGMapping currently creates a map of all tasks but it should only do so for notebook/script tasks. e.g., ignore tasks whose source is a function or a SQL script:
  
  Essentially we want to move the `pairs` iteration logic inside the `DAGMapping` constructor:
  
  https://github.com/ploomber/ploomber/blob/c87c60db954f72309b1e421a5399f9f6a426e5fe/src/ploomber/jupyter/manager.py#L185
  
  then add a condition to such iteration logic to ignore tasks if they're not of the `ploomber.tasks.NotebookRunner` class