*: Use language-specific heredoc delimiters

[issyl0]

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• I looked for singular EOS heredoc occurrences per-language that I could fix, and settled on HTML, Zig, diff and TSV.
• We’re regressing a bit on some of them in general (like CPP), so we’ll have to think of a strategy for that (some kind of lint that ignores caveats but nudges the others towards changing?). I’ll write an issue for the rest of the work, because it feels like I’m chasing a goal that will never be attainable of getting rid of all the EOS.