Latest release not found if user has other named directories in .cmdstan

[jgabry]

Describe the bug

Inside the default .cmdstan directory if the user has a subdirectory foo (really any directory name alphabetically after "cmdstan") then CmdStanR will try to set the path too foo instead of the latest cmdstan release. This is because this line sorts all names, not only ones starting with "cmdstan":

cmdstanr/R/path.R

Line 148 in e9c12be

latest_cmdstan <- sort(cmdstan_installs, decreasing = TRUE)[1]

This won't affect anyone who doesn't manually mess with the .cmdstan directory but @mitzimorris and I discovered this issue today because she had some custom directories in .cmdstan for development purposes.

To Reproduce

Add a directory foo to .cmdstan and then reload the package.

I'll make a PR to fix this.