highlight.js: Add missing overrides, update version, and run at build time

[kfranqueiro]

Changes in this PR:

• Adds language-* or no-highlight classes to code elements inside pre elements where they were missing, to provide stability across highlight.js upgrades in the event of changes to auto-detection logic
  • This intentionally does not go through the trouble/noise of adding syntax highlighting to obsolete Flash and Silverlight techniques
• Adds a build-time check which logs in dev / fails in CI if any code blocks have no class specified
  • Example of message logged for each missing class found:
    ./techniques/css/C45.html missing "language-*" or "no-highlight" class on <pre><code> starting with &lt;!DOCTYPE html&gt;\n&lt;html lang="en...
  • Example of message printed when build fails (for CI, in addition to the above message):

    [11ty] 1. Having trouble rendering liquid template ./techniques/css/C45.html (via TemplateContentRenderError)
    [11ty] 2. Please ensure all code blocks have a highlight.js class (language-* or no-highlight).
    

• Updates to use the latest version of highlight.js
• Switches to invoking highlight.js at build time and including it as an npm dependency
  • This means no more highlight.min.js bundle taking up more space in the repo on each update
  • This also means syntax highlighting no longer involves running client-side JS on every visit to every page, better optimizing for sustainability

Before switching from client-side to build-time, I verified (using the script I'd posted in #4611) that updating the bundle does not cause any syntax highlighting changes, other than adding it for the previously-missing languages.

I also performed screenshot diffs and have verified that the only changes caused by the update are fixes to syntax highlighting (primarily in terms of recognized CSS property names).

I performed screenshot diffs again after switching to build-time highlight.js, and there were only two diffs, in examples that specifically intended to include <span lang="..."> elements inside of <pre><code>. These have been adjusted to apply lang to the pre tag instead.

Note: The failure on the Check PR files check is an extremely rare false positive because I had updated it to consider script normative, but technically only script/wcag.js is involved in TR space; script/highlight.min.js is not. This should never be an issue again, since I'm removing highlight.min.js.

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