title: `dotnet sln add` wrong project type GUID
body: <p>When I do <code>dotnet sln add {myProject.csproj}</code> for a .NET Core/Standard project, it adds it as project type (I think) <code>{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}</code> instead of .NET Core's <code>{9A19103F-16F7-4668-BE54-9A1E7A4F7556}</code>.</p>
  <p>Hence, when I open the solution in Visual Studio, it complains and &quot;upgrades&quot; the <code>csproj</code> to a .NET <em>Framework</em> <code>csproj</code>, not a .NET Core one. I can manually edit the <code>sln</code> but it's a chore.</p>
  <p>Am I doing something wrong? Are there arguments I'm missing?</p>