Warn users about crossref entries in inline images

(The bug here is that ![]... needs to be on a line by itself for our crossref system to pick it up.)

Discussed in https://github.com/quarto-dev/quarto-cli/discussions/5568

^{Originally posted by MichelDSchmid May 16, 2023}
Hi all

I have a, hopefully, small problem with cross references in a book project.

![NTC-Bauformen](images/ntc_bauformen.png){#fig-bau-formen width=9.5cm}
NTCs gibt es in verschiedensten Bauformen wie in @fig-bau-formen ersichtlich ist. Diese Bauformen stellen lediglich eine Auswahl der zur Verfügung stehenden Bauformen dar.

I'm always getting the error message:
WARNING: Unable to resolve crossref @fig-bau-formen
and of course I can't see a cross reference in the document, indepentend if it's in html or pdf..

I coudn't found something in the docs and I'm using quarto version 1.3.353

Thanks in advanced
Mike