DEV: improves sk api (PR #9653)

  • reduces the API to 3 actions for now: appendContent/prependContent/onChange
  • well tested
  • removes all previous APIS which were only half supported or too dangerous as they could collide with other plugins or core behaviors
  • this plugins also puts every sk test helpers in one file


@eviltrout a little bit of context for the review. I haven’t actually updated the API when doing sk2, I wanted to see what people were actually using, from plugins, themes… My conclusion is that I have been proving a too vast API, so I have been reducing to three simple cases which should cover most use cases people have.

It shouldn’t break any external plugin, I could add backward compat if needed.

I am always happy to delete code and reduce API surface area!