Update docs and adds tests for Quarto files

[IndrajeetPatil]

Closes #1486

[AshesITR]

Not sure if that's the reason, but trim_some() is supposed to be called like this:

trim_some('
  ---
  title: "bla"
  ---

  Content
')

Mind the location of the quotes.

Also note that knitr format detection is possibly dependent on the version of knitr that is installed.

[IndrajeetPatil]

This is the problem.

For .qmd, the code in question leads to an error (and therefore 2 lints are produced):

For .Rmd, no error:

So, I can just drop this test for .qmd.

[IndrajeetPatil]

@MichaelChirico I've marked this as ready for review. I wanted to wait for issue with unexecuted code blocks to be resolved (cf. #1486 (comment)), but that is too open-ended to block this PR.