Add IsEncrypted() method to File struct

[belyalov]

It is an easy way to determine if file is encrypted - useful when you need to e.g. find and do something with password encrypted archives.

I tried to keep it simple and move this code out of data path - to make it seamless for everybody who does not care about this functionality.

[LinkLeong]

This function is very useful, I hope to deal with it as soon as possible

[bodgit]

I'm not going to merge this for a couple of reasons:

• This doesn't help in the scenario of the password being wrong and the archive headers are encrypted, in this case you won't be able to open the archive to even see if the files are encrypted.
• There is more than one encryption method for 7-zip archives, (although only the one is currently implemented as I haven't needed to support any others), so I'd rather not add a special case for just this one.

I have just merged in #278 and released in v1.6.0 support for returning a sevenzip.ReadError on certain operations. This includes a hint that encryption is involved somewhere in the stack and works for both opening the archive as well as extracting a file so hopefully covers all of the scenarios.

You can check using errors.As() with something like this:

r, err := sevenzip.OpenReaderWithPassword(archive, password)
if err != nil {
        var e *sevenzip.ReadError
        if errors.As(err, &e) && e.Encrypted {
                // Encryption involved, retry with a different password
        }

        return err
}