Add a `repair_variable_names` function

This allows you to translate from the names Stan uses (like theta.1) to those Posterior wants (theta[1]).

This function exists (privately) in cmdstanr, and is currently very simple:

repair_variable_names <- function(names) {
    names <- sub("\\.", "[", names)
    names <- gsub("\\.", ",", names)
    names[grep("\\[", names)] <- paste0(names[grep("\\[", names)], "]")
    names
}

This doesn't work for complex numbers or tuples, but since posterior doesn't seem to support those either I think this is fine

The issue names no target file or test. Start by locating the package entry point for variable-name utilities and the existing tests around Stan variable names, then compare the function's behavior with the shown examples. Done means the public function translates names such as theta.1 to theta[1] and has coverage for the supported naming cases.