FR: option to print the output of --help-all in json format

[mike-lawrence]

I'm playing with an R package that permits supplying the runtime args as a nested list, thereby making things flexible to future possible changes in what cmdstan accepts. To validate this user input (and also to show the user what's allowed), it would be useful to have the output of --help-all in an easily-parsed format like json. Or maybe there's already an easily-parsed way to get the output already that I'm missing?

[mitzimorris]

the future possible changes to CmdStan are flattening the arguments and using C++ command line parsing package cli11 - cf #925.
from that package we'll get --help for free.

the current CmdStan command argument handling is a failed experiment, and we need to move on.