Add NuGet package support

NuGet packages (commonly with file extension .nupkg) are used by .NET to
transport libraries, similar to jar files. NuGet packages are
essentially zip files with some expected contents. A mininial library
looks like this:

	$ unzip -l ClassLibrary.0.0.1.nupkg
	Archive:  ClassLibrary.0.0.1.nupkg
	  Length      Date    Time    Name
	---------  ---------- -----   ----
	      502  04-14-2025 18:14   _rels/.rels
	      407  04-14-2025 18:14   ClassLibrary.nuspec
	     3584  04-14-2025 22:14   lib/net8.0/ClassLibrary.dll
	      459  04-14-2025 18:14   [Content_Types].xml
	      625  04-14-2025 18:14   package/services/metadata/core-properties/b44ebb537bbf4983b9527f9e3820fda6.psmdcp
	---------                     -------

Some of these content are similar to the contents of a Microsoft OOXML
file, and `file` utility doesn't recognize this correctly. Eee
https://bugs.astron.com/view.php?id=644

Support NuGet packages in diffoscope will make it easier to verify the
.NET SDK and .NET libraries. It will also contribute towards making the
.NET SDK itself reproducible (see
https://github.com/dotnet/source-build/issues/4963).