[rustdoc] Add --output-format=doctest command-line flag

[GuillaumeGomez]

Part of #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).

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