Ratatui

[mo8it]

I would like to use Ratatui in Rustlings, especially for the watch mode.

Why

• The current state of the watch mode is very rigid. For example, once you resize your terminal, your progress bar is broken.
• The progress bar is often hidden and you need to scroll up to look at it. This is a problem because you need the information of the progress bar to know what exercise you need to start next. With Ratatui, we can make the progress bar always visible as a fixed widget at the top.
• The kind of REPL in watch mode is hidden and not intuitive. For example, you only find out about the ability of typing hint if you read the whole message when you started the watch mode. And still then it is not intuitive that you can just type a command and receive some output because you normally don't do so when a program is spitting out some output. Instead, we can introduce shortcuts for the different commands in the watch mode and show them in the footer.
• Offer a better UX for people that aren't very familiar with the terminal because they get a UI.
• The ability of adding more features. For example, the list of all exercises with their status could be shown in an additional tab. Then we can offer the functionality of skipping exercises in this interactive exercises list. This is something that could have been very helpful during my course when a student falls behind a couple of exercises. They were not able to use the watch mode anymore.
• Demonstrate the ability of building awesome TUIs in Rust :D

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@shadows-withal what do you think? I would take care of the implementation and put it under a 6.0 milestone.

[mo8it]

Done in #1942