detect missing ids when prerendering

[Rich-Harris]

Closes #3955. This throws an error in the case where a page links to /a#b and /a doesn't include an element with id="b".

We probably want to make that behaviour configurable somehow, since some people might want a warning rather than an error, or even custom error handling. For normal prerendering errors this is enabled via config.kit.prerender.onError, but the API can't accommodate these errors. I'm not really sure what to do about that, so I'm just going to leave this in draft for now.

Also not pictured: tests of funky IDs that involve URL or HTML encoding. No idea if that works in this branch currently but either way it needs tests.

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[Rich-Harris]

The deploy failure is because this is working — it's identified an incorrect link

[Rich-Harris]

This replaces config.kit.prerender.onError with config.kit.prerender.handleHttpError, to distinguish it from the new config.kit.prerender.handleMissingId.

Remaining question is whether the default for config.kit.prerender.handleMissingId should be fail or warn. Thoughts?

[dummdidumm]

Apart from a minor naming (which I'm ok with if you turn it down) thing this looks good!

[dummdidumm]

I was confused for a moment because I understand a regular link to not just the home page by "deeplink", I think something like "hashlink" would be better.

[Rich-Harris]

agree — fixed