Support for emitting custom warnings

[ondohotola]

Description

In issue #1276 @Enivex proposed a very helpful solution for missing references.

However, I noticed that that will happily generate (?) (which I have adapted to ?? without reporting this, which means you may end up with orphan references.

I have not been able to find a way to make typst squeal so I can adapt the #show raw rule's missing condition.

Use Case

There should be other conditions where a warning type message triggered programatically might be helpful.

[laurmaedje]

Related: #600