instantiate: pre-compiled CmdStan models in R packages

Similar to rstantools for rstan, the instantiate package builds pre-compiled CmdStan models into CRAN-ready statistical modeling R packages. The models compile once during installation, the executables live inside the file systems of their respective packages, and users have the full power and convenience of CmdStanR without any additional compilation after package installation. This approach saves time and helps R package developers migrate from rstan to the more modern CmdStanR.