`Zipline`'s `quickJs` property and `QuickJs` type should have an opt-in annotation

[JakeWharton]

Something to indicate you're mucking around with the low-level engine which is dangerous, can break Zipline, and isn't the engine we want to commit to long-term. It's an implementation detail that we're required to expose publicly because there are very good reasons for accessing it, but you still need to take care.