feat: Rewrite default out text

This has been in place for a long time now, before we had an install
script, so it ended up repeating a bunch of the same things that the
install script does automatically. I rewrote it so that it gives more
helpful information about how you're supposed to do Rustlings.
Hopefully this will reduce the number of "I started Rustlings and it
gave me an error" issues (no offense to anyone who opened one of those,
it was pretty unclear that it _wasn't_ an error).

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