[syphar]

So, the idea is:

• we call the cargo build with --message-format=json.
• but we only store the parsed messages on the side, dropping everything except compiler-messages.
• the normal build logs still get the rendered version and don't see any json
• we store the extracted messages in a separate build log next to the normal build log

For now, the goal for RawCargoMessage is just to do small checks on the json message, and then emit it for the .jsonl log. Later we can analyze that, or change the build-log view to render them nicely.

( this could also be an extension to rustwide, but it works like this too)