.SLNX format support

[surayya-MS]

Fixes #10266

Context

We currently use our own parser SolutionFile.Parse() to parse .sln files in order to build them.
In this PR I intergrate the new package Microsoft.VisualStudio.SolutionPersistence to parse the new format .slnx files.

Key differences in the SolutionFile from the package's SolutionModel:

1. ProjectsInOrder and ProjectsByGuid no loner include solution folders because they are saved separately (in the package SolutionModel.SolutionProjects and SolutionModel.SolutionFolders)
2. SolutionConfigurations are not available in the SolutionModel, instead there are BuildTypes and Platforms. Since SolutionConfigurations are used in SolutionGenerator a lot, I needed SolutionConfigurations represented accurately. Combination of SoutionModel's BuildTypes and Platforms might be redundant, so I collect Solution configurations from each project's configurations.

Changes Made

Testing

1. Added tests in SolutionFile_Tests that cover .slnx cases
2. Added SolutionFile_NewParser_Tests with only one test for parse website properties as it was not possible to do that in SolutionFile_Tests because AspNetConfigurations is internal.
3. Tested manually as well.

Notes

[ssg]

I propose explicit matching of filename extensions.

[kasperk81]

can replace existing .sln serializer with https://github.com/microsoft/vs-solutionpersistence/tree/main/src/Microsoft.VisualStudio.SolutionPersistence/Serializer/SlnV12

[rainersigwald]

@kasperk81 we're going to use that deserializer in a follow-up PR. We're trying to make this PR the "minimum risk for potential backport to 9.0.1xx SDK" change, so keeping the existing sln behavior.

[JanKrivanek]

All looks as expected.

But I did just a quick pass - will go more in depth tomorrow

[JanKrivanek]

Thank you!

[kasperk81]

@surayya-MS should fix tests for unix handling be done in the product or does it produce expected style understood by consumer code?

[surayya-MS]

@surayya-MS should fix tests for unix handling be done in the product or does it produce expected style understood by consumer code?

It's just / or \ difference.

[kasperk81]

Opened dotnet/source-build-externals#392 for source build error (should be the same version as the package)

[surayya-MS]

follow-up PR to replace existing .sln parser with Microsoft.VisualStudio.SolutionPersistence #10836