FIX: Do not highlight large code blocks (PR #10125)

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This is good, though we should probably add a test

I did not succeed at first to do that because highlight.js is loaded at run-time. I gave it another try and found out a test which was doing something similar, so I used that as a base.

The "puts a\n".repeat(15000) might be time-consuming / CPU-intensive. I think we should only do it once and put it in a const?