Use dbg! instead of println! for simple debug prints

[mgeisler]

Originally, we did not show stderr output from the Playground programs. This was fixed in #2397 and so we can finally start converting the trivial debug print statements to use dbg!.

Specifically, I'm thinking of the many print statements that look like this:

println!("a: {a:?}");

They can simply become

dbg!(a);

When doing this conversion, pay attention to any <!-- i18n:skip --> lines before the code block. They were added to skip translation of the "a: {a:?}" string, but now that the string disappears, the skip directive should be cleaned up too.