[rustdoc] Add `--extract-doctests` command-line flag Part of https://github.com/rust-lang/rust/issues/134529. It was discussed with the Rust-for-Linux project recently that they needed a way to extract doctests so they can modify them and then run them more easily (look for "a way to extract doctests" [here](https://github.com/Rust-for-Linux/linux/issues/2)). For now, I output most of `ScrapedDoctest` fields in JSON format with `serde_json`. So it outputs the following information: * filename * line * langstr * text cc `@ojeda` r? `@notriddle`