Use SlnGen to generate solution from Traversal project

[max-charlamb]

Updates repository to use Microsoft.Build.Traversal .proj files to drive which projects are built. Previously the arcade build infrastructure would build the solution files found in the root directly. This approach gives us more fine-grained control over projects to build, as well as allow us to automatically generate a solution file using slngen.

I have checked in the generated build.sln which can be loaded with VS or VSCode. This version includes the vcxproj files which improve the native debug experience in VS. There is an option to generate build.sln without these files as they can cause warnings on non-Windows platforms (same behavior as today).

For convienence I have added two new scripts:

• eng/generate-sln.ps1
• eng/generate-sln.sh
  both of these automatically invoke the proper commands to regenerate the build.sln file.

I updated the instruction docs to reference build.sln and explain how to regenerate it.

Edit:

• I have reverted from using slnx -> sln as the conversion process causes some problems with the vcxproj Ids. Given that these are now auto-generated, using the sln format seems acceptable as it has broader support.

[hoyosjs]

From a technical perspective, I like the traversal better. That being said:

• Consider a tools manifest to lock versions
• this loses the native file view and the sln folder view. I can't say I used a lot, not sure if everyone feels that way

[steveisok]

I think if there is a need for the prior .sln format, we can easily bring it back. I suspect it's just easier to manage .slnx files, but AI is also good at structuring the .sln file for you.

[hoyosjs]

Thanks :) Much easier to reason about and still have a reasonable loop