Add exercises about help mode and shell mode

[fmarotta]

Add exercises for the REPL lesson:

• One exercise for help mode about reading the documentation of the varinfo() function
• Two new exercises for shell mode, one about pwd and cd, the other about ls

[kescobo]

I wonder if it would be worth comparing / contrasting with cd() and readdir() in julia - using the shell is great for quick things, but you can't then assign things to variables or run julia functions on the outputs.

Or maybe that's an added complication that's not worth it at this stage of the lesson

[BeastyBlacksmith]

We could mention it in an info box or similar

[BeastyBlacksmith]

but can also be in a different PR

[kescobo]

I think this is a nice addition 👍

[BeastyBlacksmith]

@all-contributors add @kescobo for review and @fmarotta for content

[allcontributors]

@BeastyBlacksmith

I've put up a pull request to add @kescobo! 🎉

[BeastyBlacksmith]

@all-contributors add @kescobo for review
@all-contributors add @fmarotta for content

[allcontributors]

@BeastyBlacksmith

I've put up a pull request to add @kescobo! 🎉

[BeastyBlacksmith]

@all-contributors add @fmarotta for content

[BeastyBlacksmith]

Sorry for spamming 😅, figuring out how this works

[BeastyBlacksmith]

@all-contributors add @fmarotta for content

[allcontributors]

@BeastyBlacksmith

I've put up a pull request to add @fmarotta! 🎉