Consider adapting a typestate-ish pattern for context manager-based components

[janbuchar]

It would probably be hard to do so without straying even further from how the JS implementation works, but it may be easier to just do something like

class EventManager:
  async def __aenter__(self) -> EventManagerContext:
    pass
  # ... no methods that need to be called on an initialized event manager

class EventManagerContext:
  def emit():
    pass
  # and whatever else needs an initialized context manager

... and so on for everything with an initialization step. Honestly, even BasicCrawler could benefit from this - the tracking of active state is kinda hairy there as well.

Originally posted by @janbuchar in #714 (comment)