Add support for missing references (w/ warnings)

[m-krastev]

Description

Coming from LaTeX, you could compile a document even if a reference is not defined. This would trigger a warning and a "??" would be printed on the resulting file.

Currently, Typst just errs and exits the whole compiler process (e.g. if using typst watch).

Use Case

A similar functionality where the error is ignored and a warning is emitted instead would help creative flow while writing so that one does not have to immediately go back to a missing reference and try to address it.

[Enivex]

See #1276 and #1322

[m-krastev]

I would love for the functionality of #1276 to be included in the base compiler, as this should not be considered as a critical compiler failure.

[laurmaedje]

If we can come up with a good design, I'm open to it.

[Coekjan]

Maybe we can add a cli argument (like --allow-missing-ref, or something else) to toggle the compile/watch behavior:

1. normal (current) behavior: deny missing ref
2. --allow-missing-ref behavior: warn on missing ref, and print ? (or something else, or something configurable by cli args) in the result

[LaurenzV]

I think instead of having a specific flags just for refs we should instead think about how to implement a more generalized system for enabling/disabling warnings, then this could be extended to this specific use case as well.

[geajack]

Yes! You want to be able to just put the references in as you're writing and add the actual reference targets at the end. An --ignore-refs flag or something would be ideal. The current behavior encourages putting in a bunch of dummy referands just to get the compiler off your back, which is worse since you might forget them.

This at the beginning of the document makes a good workaround:

#show ref: it => {
  it.target
}

However, this doesn't solve the problem because now all refs will be replaced by a placeholder, even ones that work, so you can't gradually fix broken refs and have them fill in.