[enchancement] Skip the download if .zip file exists and passes checksum check

[Pretzelek]

Hello
I think it would be great if the app would skip download if the .zip file (created with the -zip option) exists and is valid.
This would make keeping the content up to date with new entries.

[lasers]

I'd like something like this, but verification could be done using zipped pkgs instead of extracted zips... based on checksum. That way, the zip files could be named anything, but pyNPS would be smart enough to not care about that and just check the pkgs themselves in the zips.

Example:

❯ unzip -l "PKG/PSV/GAMES/Random Game [PCSA01234] [USA].zip"
Archive:  PKG/PSV/GAMES/Random Game [PCSA01234] [USA].zip
  Length      Date    Time    Name
---------  ---------- -----   ----
800000000  05-31-2023 06:00   xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.pkg
---------                     -------
800000000                     1 file