Separate installation from configuration

[sanmai-NL]

Currently, the installation script installs the pnpm executable as well as sets up shell configuration. Sometimes, PNPM shell configuration must be managed by the user (e.g., container image builds where PNPM is installed under an unprivileged user with a nologin shell). Offer command line options to customize the actions of the installation script, and by default, don't alter configuration.