Support embedded PCKs

[ithinkandicode]

The small utility program GodotPCKExplorer can extract an embedded PCK from its EXE.

And as @Qubus0 discovered, it also has CLI support.

Could we add support for doing this as part of the setup process?

The CLI args aren't explicitly documented, but they can be found in the source code -- see here (ctrl+f: "-e").

Note: This can come after PR #81 #89, which doesn't need to be held up by this implementation

[ithinkandicode]

@KANAjetzt raised an issue to request this feature be added to PckTool: hhyyrylainen/GodotPckTool#26

[KANAjetzt]

I'll close this for now because we currently don't have a use case for it.