Option to change the default output directory of generated files for quarto use binder

[chainsawriot]

Description

The current default is to add the generated files (e.g. PostBuild, runtime.txt) to the root directory . of the project. repo2dockersupports these files to be in ./.binder or ./binder (an example and documentation).

It would be nice if we can change the default directory from the root directory to ./.binder or ./binder. It separates cleanly what files are generated by quarto for binder.

[cderv]

Thanks ! I moved this to #9313 as it should be easy enough to add unless I am missing something. Less follow there.