Use `std::ops::ControlFlow` in ‘extract function’ assist

[lunacookies]

Currently when triggering ‘extract function’ on a section of code that contains break, the assist will make the extracted function return a boolean to indicate whether the caller should break out of the loop. Now that std::ops::ControlFlow has been stabilised, it might be better to return ControlFlow::Break if the caller should break, and use if let instead of let in the caller.