Convert Listings in Chapter 06 to `<Listing>`

[SpectralPixel]

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Finally, here is chapter 06 adapted to the new <Listing> pre-processor. I'm not sure if everything works as intended since I couldn't get mdbook to behave properly on my machine, so please check the sanity of my changes.

I also changed numberless listings (as in, they have a file name, but no number), so feel free to revert those commits if they are not yet fully functional by the time they will be merged.

Please also check all the comments I have left in commit descriptions and the issue thread for further concerns I might have about the code.

(By the way, I also fixed some conflicts while moving the commits around)

Issue #3919

[chriskrycho]

Thanks! Looks like you have at least one unclosed <Listing>! Let me know when you have it fixed and I’ll do a review pass!

[chriskrycho]

Ah, I see your last commit – that one is the problem, I expect. HTML blocks (like these, but also just like the old <span>-based versions) are not subject to the same rule, and Markdown parsers generally don’t handle wrapped tags well.