fix(blog): ignore hidden code blocks when calculating read time

[LeoMcA]

Summary

Problem

Hidden code blocks were being included in the read time calculation for blog posts, which leads to https://developer.mozilla.org/en-US/blog/scroll-progress-animations-in-css/ for instance having a 25 minute read time, because of lots of lorem ipsum being included in the examples.

Solution

Ignore hidden code blocks when doing this calculation: the above article now has a 7 minute read time.

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How did you test this change?

• yarn dev, visited article locally
• also pasted source of article into https://regex101.com/ and checked my regex matched the intended blocks (and nothing else - I needed that non-greedy match ?)

[bsmth]

Thanks, Leo - looks good to me 👍🏻