Add support for `Microsoft.Build.Traversal` projects

This one might be a bit of a reach, but I've heard from many people that they would like traversal projects to act as solutions. Personally, it's the ease with which projects can be included. Traversal projects could make use of the familiar MSBuild properties, items, targets, and tasks known from project files.

<Project Sdk="Microsoft.Build.Traversal/4.1.0">

  <ItemGroup>
    <ProjectReference Include="src/*/*.csproj" />
  </ItemGroup>

</Project>

I'm curious about people's alternative ideas, since I'm not optimistic that this can be supported. MSBuild projects are a lot more complicated, and using one as a solution would probably produce unexpected behavior.

I would still like to open the discussion for some format that's closer to MSBuild project files, so let me know if you have any ideas on how to perhaps use the MSBuild architecture to build a solution format that integrates with MSBuild. A possible format I've imagined is the following:

<Project Sdk="Microsoft.Build.Solution">

  <ItemGroup>
    <Configuration Include="CustomConfiguration" Platform="Any CPU" />
  </ItemGroup>

  <ItemGroup>
    <Project Include="src/*/*.csproj"
             Folder="src" />
    <Project Include="test/*/*.csproj"
             Folder="test" />

    <Project Update="test/*/*.csproj"
             Condition="'$(Configuration)' == 'Release'"
             NoBuild="true" />
  </ItemGroup>

</Project>

Start by reading the repository's solution serializers and models, then compare their supported solution behavior with the proposed Microsoft.Build.Traversal and Microsoft.Build.Solution XML examples. The issue does not identify an entry point, tests, or an agreed format; done would require a decided design and corresponding support.