Add cachebusters to CSS and JS

[SquidDev]

Just adds a ?v=filehash to the CSS and JS files. This does not apply to Agda-only files (so those using the "code" template), but I think that's probably fine for now.

This does massively increase build times when changing CSS. It looks like some markdown files are taking >5 seconds to be converted. I'm not sure why this is, want to do some profiling before merging.

[SquidDev]

Adding a couple of timedM calls, shows we're spending a lot more time rendering Markdown than I'd expect.

Label	Time (s)
Check markup	0.71
Fix markup	4.12
Parse Markdown	4.25
Patch markdown	0.46
Render Markdown	21.25
Search	0.29
Write file	2.77
patchInline	0.46
patchInlines	0.36
uncommentAgda	0.32

Not sure why this is - I'd expect Pandoc to be relatively sensible here. Might try to see if we can build a profiling build of 1lab.

[SquidDev]

After some profiling, most of the time was being spent inside Skylighting (I was running with --skip-agda). I've patched Skylighting to pre-compute the regexes, ensuring they're cached between code blocks, which brings the total build time down to ~7s (from ~15s) - not ideal, but I think workable now.

Not 100% wild about the maintainability of this - I will submit this patch to upstream, so we may want to wait if/until that is merged.

[github-actions]

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