Add /help command in REPL

[perlun]

The current way the REPL works is not very helpful. You don't get a whole lot of information from it on how it should be used; it doesn't even tell you how to exit it! (Now, that's "semi-easy" to guess since both quit and exit works, but still...)

We should add a basic /help command which displays something like the following:

• /quit and /exit, and potentially other internal REPL commands
• Some examples of statements/expressions that works in the REPL
• Preferably, a pointer (URL) to some tutorial-oriented documentation that can be kept always-up-to-date.