Set `Prism.manual = true` to prevent unnecessary code execution

[JaKXz]

Originally posted in #6688 (comment)

Here are Prism's docs on the issue: https://prismjs.com/#basic-usage and a bit longer explanation in source

TLDR Prism has a legacy assumption that you're going to render your code statically and so it calls its highlightAll method under the hood to replace everything with the tokenized spans after JS has loaded. I think it searches for elements matching the .prism-code selector, so that's probably why it hasn't been noticed but the code is still being executed unnecessarily.

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