Optimize UI responsiveness in scanning state

[unixzii]

The UI is very laggy while the scanning is in progress, and it makes the program almost unusable during the first few seconds of startup. The bottleneck is jwalk iterator, it may block the UI thread sometimes. In this PR, I alleviated the situation by some optimizations, and the UI is now super responsive at startup.

Here is what it looks like:

https://github.com/Byron/dua-cli/assets/3109144/3b06b888-c4ec-4c45-9468-36b0059cbe6c

Some implementation notes:

• The traversal operation is moved to another thread to prevent the UI thread from being blocked, and its results are sent to the UI thread via a channel.
• Use crossbeam::channel to receive UI events and jwalk entries. In contract to std::sync::mpsc, we can select from a set of channels without busy polling using try_recv.
• Created our own version of input_channel. The function provided by crosstermion returns a std::sync::mpsc::Receiver value, which is not compatible with crossbeam::select!.

[gosuwachu]

I have tested this change and indeed it is now super smooth 🎉 Thanks!

[Byron]

Thanks a million, it works just as advertised!

It's so good to have this issue finally fixed!

And because we had to wait so long for this to be improved, I didn't want anybody to wait even longer: here is the new release