Execution: Add stderr block

[Alx-Lai]

Add a stderr block.
Pros:

• we can make use of dbg macro in our code Cons:
• there's a limitation that the compile message also shows

Applies patches from rust-lang/mdBook#1315 since the original change was not merged by rust-lang.

Issue: #531

[mgeisler]

Hi @Alx-Lai, oh wow, this patch looks great! Thanks a lot for looking into this, I would very much like to have this feature implemented.

I gave the PR a quick test run locally and this is how it looks:

Screencast.from.2024-10-06.23-12-20.mp4

This is nearly perfect. A few comments:

• Could you match and remove the three lines that appear when things compile correctly? The lines which say:

     Compiling playground v0.0.1 (/playground)
      Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.60s
       Running `target/debug/playground`
  

  By removing them in the happy case, we end up with the same output as today, which helps limit the amount of vertical space used. This is important for a live class where it's very disturbing to scroll up and down :)

• Could you avoid showing the stderr block immediately when compilation starts? Ideally, there would be a single output with the combination of stdout and stderr, but I understand that the Playground delivers the data in two separate fields.

• Similarly, the block with the stderr output should be hidden/cleared when the code is recompiled.

I looked briefly at the full Playground and discovered that they use a WebSocket now to stream the output line by line! They include markers to show if this is a stdout or stderr:

I would be happy to merge this PR as a first step — I don't know how difficult it will be to adapt the mdbook code to use a WebSocket as well.

[Alx-Lai]

Ok, I'll spend time looking into it. Thanks for the feedback!

[Alx-Lai]

Updated the commit.

• Remove compile message with a not-that-elegant way, the compile error message would not be replaced.
• hide and clear stderr block before compiling the code.