ILSpyCmd: Add support for decompressing single-file bundles

[Freakness109]

Add a new flag -d|--dump-package to ILSpyCmd that decompresses
single-file bundles into a target directory.

Fixes #2498

Problem

#2498: Could not handle single-file bundles

Solution

• Files in a single-file bundle are extracted into a directory, where subsequent runs of ILSpyCmd can actually decompile the assemblies.
• Tried to stay consistent with the code in ILSpy/LoadedPackage.cs, specifically the functions FromBundle and BundleEntry.TryOpenStream.
• This patch does not handle the case of subdirectories in single file bundles (I'm not sure whether they are allowed).
• I'm not sure how to test the change

[siegfriedpammer]

Thank you very much for providing this feature! Sorry for taking so long to review this!