`rix_init()` implement "quiet" and improve "simple" message_type

[philipp-baumann]

There is a new message_type = "quiet" option which writes rix init .Rprofile without any message to the user. Also, message_type = "simple" is a bit slimmer.

Simple (default)

> rix_init(message_type = "simple")

### Bootstrapping isolated, project-specific, and runtime-pure R setup via Nix ###

==> Existing isolated nix-R project folder:
 /Users/philipp/git/philipp-baumann-shared/rix 

==> R session running via host operating system or docker

* Keep existing `.Rprofile`. in `project_path`:
 /Users/philipp/git/philipp-baumann-shared/rix/ 

==> Also adjusting `PATH` via `Sys.setenv()`, so that system commands can invoke key Nix commands like `nix-build` in this RStudio session on the host operating system.

Quiet: as expected, doesn't message

> rix_init(message_type = "quiet")

Verbose: output messages help to understand the working principles verbosely:

> rix_init(message_type = "verbose")

### Bootstrapping isolated, project-specific, and runtime-pure R setup via Nix ###

==> Existing isolated nix-R project folder:
 /Users/philipp/git/philipp-baumann-shared/rix 

==> R session running from RStudio

* Keep existing `.Rprofile`. in `project_path`:
 /Users/philipp/git/philipp-baumann-shared/rix/ 

* Current `PATH` variable set in R session is:

/usr/local/bin:/System/Cryptexes/App/usr/bin:/usr/bin:/bin:/usr/sbin:/sbin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/local/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/appleinternal/bin:/Applications/quarto/bin:/Library/TeX/texbin:/usr/texbin:/nix/var/nix/profiles/default/bin:/Applications/RStudio.app/Contents/Resources/app/bin/postback

==> Also adjusting `PATH` via `Sys.setenv()`, so that system commands can invoke key Nix commands like `nix-build` in this RStudio session on the host operating system.

* Updated `PATH` variable is:

 /usr/local/bin:/System/Cryptexes/App/usr/bin:/usr/bin:/bin:/usr/sbin:/sbin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/local/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/appleinternal/bin:/Applications/quarto/bin:/Library/TeX/texbin:/usr/texbin:/nix/var/nix/profiles/default/bin:/Applications/RStudio.app/Contents/Resources/app/bin/postback

* Current lines of local `.Rprofile` are:

### File generated by `rix::rix_init()` ###
# 1. Currently, system RStudio does not inherit environmental variables
#   defined in `$HOME/.zshrc`, `$HOME/.bashrc` and alike. This is workaround to 
#   make the path of the nix store and hence basic nix commands available
#   in an RStudio session
# 2. For nix-R session, remove `R_LIBS_USER`, system's R user library.`.
#   This guarantees no user libraries from the system are loaded and only 
#   R packages in the Nix store are used. This makes Nix-R behave in pure manner
#   at run-time.
{
    is_rstudio <- Sys.getenv("RSTUDIO") == "1"
    is_nixr <- nzchar(Sys.getenv("NIX_STORE"))
    if (isFALSE(is_nixr) && isTRUE(is_rstudio)) {
        cat("{rix} detected RStudio R session")
        old_path <- Sys.getenv("PATH")
        nix_path <- "/nix/var/nix/profiles/default/bin"
        has_nix_path <- any(grepl(nix_path, old_path))
        if (isFALSE(has_nix_path)) {
            Sys.setenv(PATH = paste(old_path, nix_path, sep = ":"))
        }
        rm(old_path, nix_path)
    }
    if (isTRUE(is_nixr)) {
        current_paths <- .libPaths()
        userlib_paths <- Sys.getenv("R_LIBS_USER")
        user_dir <- grep(paste(userlib_paths, collapse = "|"), current_paths)
        new_paths <- current_paths[-user_dir]
        .libPaths(new_paths)
        rm(current_paths, userlib_paths, user_dir, new_paths)
    }
    rm(is_rstudio, is_nixr)
}