Let extra_syntaxes_and_themes reference builtin syntaxes

[9999years]

Closes #1607
Closes #1678

This lets syntax definitions in extra_syntaxes_and_themes reference builtin syntaxes (by scope: or include: or similar).

Unfortunately, re-building the SyntaxSet is rather expensive. On my (small) personal website, this makes Zola render my site in 469ms rather than 63ms — a slowdown of nearly 8x!

We could potentially mitigate this by serializing the computed SyntaxSet to a file, for example with a cached::DiskCache, but I don't see existing caching infrastructure so I'm not sure if that's a good tradeoff.

Is this wanted? The performance cost is not worth it with the current implementation, but maybe we can optimize it?

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[Keats]

That's a huge slowdown. Let's see if someone from syntect can weight in because that's too much IMO

[9999years]

@Keats Yeah, agreed. I opened an issue upstream, we'll see what they think: trishume/syntect#562