do not allow !expr tags outside of knitr execution

[cscheid]

Closes #11967.

The new error message is now:

$ quarto render docs/yaml/issue-11967.qmd
ERROR: In file docs/yaml/issue-11967.qmd
(line 4, columns 8--35) !expr tags are not allowed in Quarto outside of knitr code cells.
3: format: html
4: brand: !expr 'c("demo/_brand.yml")'
         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
5: ---