Provide compressed length information for content files

[vers-one]

Provide compressed length information for content files

This is:

• a bug fix
• an enhancement

Related issue: #144

Description

This pull request adds the CompressedLength property to the IZipFileEntry interface. The new property represents the compressed size of the entry in the ZIP archive.

Testing steps

Use the following code to test the new property:

string filePath = ...
using (EpubBookRef bookRef = EpubReader.OpenBook(filePath))
{
    string firstHtmlEntryFilePath = bookRef.Content.Html.Local[0].FilePath;
    IZipFileEntry firstHtmlEntry = bookRef.EpubFile.GetEntry(firstHtmlEntryFilePath)!;
    Console.WriteLine($"FilePath: {firstHtmlEntryFilePath}");
    Console.WriteLine($"Length: {firstHtmlEntry.Length}");
    Console.WriteLine($"CompressedLength: {firstHtmlEntry.CompressedLength}");
}