[FEAT]: Add ability to parse new SLNX format

[wgnf]

Description of the feature/enhancement

Current .NET 9 previews contain early versions of the new slnx Solution format (XML). SlnParser should also support that format, so the parsing should be extendet for that.

Additional Information

No response

[wgnf]

I've seen some Microsoft-build SLN/SLNX Parser lately, maybe I have to look into getting that integrated... 🤔

[wgnf]

That is what I've meant: https://github.com/microsoft/vs-solutionpersistence
Might be good to use to be resilient towards future changes.

[wgnf]

BUT thinking about it... I'd argue that is a good thing that SlnParser currently has no references to other packages.

Usint the above mentioned package would break this "feature"

[wgnf]

I've gone ahead and implemented SLNX support for Cake: cake-build/cake#4666

Maybe something similar can be done here as well. Some things I've noticed while working on the contribution there:

• There aren't any IDs anymore, so we'd have to omit them
• Type-IDs are not given, unless it is not the default C# class-library type id
• Nested folders are supported, but are not given in a nested form in the SLNX file. I don't know yet if that is important. It is still an open discussion in the Cake contribution as well