Swiftly installation should check for a tty and not prompt for confirmation if there isn't one

[jakepetroules]

The Swift installation instructions on swift.org contain the following:

./swiftly init --quiet-shell-followup

However, when run in environments where there is no tty (like a Docker container), it still prompts for confirmation. We should detect that there's no tty and continue anyways, so that the same instructions for installation work both for interactive and non-interactive use cases.

[louisunlimited]

I think we want to use --assume-yes flag instead in a environment like that? The only thing that prompts during the init process are aborting installation, which can be avoided with this flag.