feat: Give you the option of running R scripts in the folder in which they are stored.

[Cervangirard]

The aim would be to give you the opportunity to launch the R script in the folder where it is located, as would be the case for a Rmarkdown, for example.

That could be possible if we change the behaviour of this quarto_execute_dir

[akselthomsen]

In #124 I have implemented a new option execute_dir where you can either specify a character path to be used for all scripts, or a function, that given the path to the script returns the appropriate execute dir.