Add folder check in move-to-archive

[mergen3107]

First try.
Function is taken from: https://www.geeks3d.com/hacklab/20210901/how-to-check-if-a-directory-exists-in-lua-and-in-python/

Use case:

1. Open KOReader with MoveToArchive plugin enabled, setup folder for it.
2. While KOReader is open, rename that folder to something else.
3. Try to go to that folder from the plugin menu. Now instead of crashing, it should ask about setting a new folder for MoveToArchive.

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This change is 

[mergen3107]

Supposed to close #11245
Please let me know:

1. If there is an existing function to check if the folder exists that I should use instead of this new function.
2. If this is a robust way to do this.
3. If can add this somewhere else as well? News Downloader, and other plugins?

I tested quickly on Kindle Scribe.

Thanks!

[mergen3107]

(OK, CircleCI stopped complaining, I hope I did it right... :D)

[mergen3107]

Thank you guys! Is this way better? :D
If so, I can go around and check other plugins/places where this could be handy.