feat: ZipArchive::root_dir and ZipArchive::extract_unwrapped_root_dir

[WilliamVenner]

This PR implements the concept of a "root directory" in the Zip archive in the context of extraction.

It's very typical to find Zip archives that consist of a single, sole, top-level, root directory containing the rest of the archive's entries. In some cases this might be something the developer wants to "unwrap" when extracting.

Example

Imagine a Zip archive with the following structure:

root/file1.txt
root/file2.txt
root/sub/file3.txt
root/sub/subsub/file4.txt

If the archive is extracted to foo using ZipArchive::extract, the resulting directory structure will be:

foo/root/file1.txt
foo/root/file2.txt
foo/root/sub/file3.txt
foo/root/sub/subsub/file4.txt

If the archive is extracted to foo using ZipArchive::extract_unwrapped_root_dir, the resulting directory structure will be:

foo/file1.txt
foo/file2.txt
foo/sub/file3.txt
foo/sub/subsub/file4.txt

Example - No Root Directory

Imagine a Zip archive with the following structure:

root/file1.txt
root/file2.txt
root/sub/file3.txt
root/sub/subsub/file4.txt
other/file5.txt

Due to the presence of the other directory, ZipArchive::extract_unwrapped_root_dir will extract this in the same fashion as ZipArchive::extract as there is now no "root directory."

[WilliamVenner]

I think that should do it.