Add new debug step to knitr capabilities

[cderv]

This PR aims to help debug further #6960 by

• Separating checking R binary is working (using RScript --version) and checking R configuration (knitr and rmarkdown package).
• This allow to throw "Unable to locate R error" only when R found by Quarto is really not working, and throw other error regarding capabilities otherwise.
• It also allow to add more debug line for when $QUARTO_LOG_LEVEL=DEBUG is set

[cderv]

Review from call with Carlos

[cderv]

@jjallaire do you know if some of our other tools / products is parsing some error message or results of quarto check ?

I would like to do the same as checkKnitrInstallation() in printCallRDiagnostics() . However, printCallRDiagnostics() is called if there is an error happending while rendering with knitr engine, so not sure about the impact of changing the info message we print.

Do you think it is safe ?

For now, this PR only modifies checkKnitrInstallation() to be safe as this is needed for #6960 but I plan to update printCallRDiagnostics() if possible later.

thanks

[jjallaire]

No one that I know of is parsing that output so I think it is safe.