Interactive canonicalization

[avehtari]

Currently cmdstan_model() outputs deprecation warning or syntax error and recommends running stanc, which is a completely foreign thing for R users. Improve the message to recommend using $format(..., canonicalize = TRUE, overwrite_file = TRUE).

Also if the session is run interactively, ask the user whether they would like to do canonicalization right away [Y/n], and then run $format(..., canonicalize = TRUE, overwrite_file = TRUE)

Also provide instructions how to make conversion for a larger number of Stan code files.

[jgabry]

I changed the name of this issue now that we have a better message in #852