Properly handle files metadata

[martinRenou]

Today, untarjs's output is simply:

export type FilesData = { [filename: string]: Uint8Array };

This fails to represent a proper files definition as we're missing metadata information about files (mtime, atime, ctime, symlink, size etc).

We should have a more advanced files representation and retrieve everything we need, not the content only.

This should be part of a breaking change release where the API changes quite a bit