Add pending UI

[senekor]

This is a POC of this suggestion. It's a little hacky, but as far as I understand it will be removed again once the issue with rust-analyzer is fixed. I think removing it is a good idea, because rust-analyzer is working fine for me and the pending UI is only shown for a very short amount of time.

Also, I added the pending UI at the very bottom, because if rust-analyzer isn't running at all the UI becomes very janky if any of the other text changes or moves.

[mo8it]

Thanks, but there is no need to keep the a pending state. I solved by only adding this:

rustlings/src/watch/state.rs

Lines 54 to 58 in 4ce8667

	writeln!(
	self.writer,
	"\nChecking the exercise `{}`. Please wait…",
	self.app_state.current_exercise().name,
	)?;

I will also keep it even after the Rust-Analyzer issue is resolved. It gives the user instant feedback which can be relevant on older PCs or with exercises with a long runtime.

[senekor]

Much better 👍