Move to slnx (#374)

This is the modern solution format for .NET. The advantages are that it
has a defined format and more resistant to merge conflicts. This is
supported by all modern .NET Tools (VS and VS Code)

Author: jaredpar

dotnet/GitHub.Copilot.SDK.sln

@@ -0,0 +1,13 @@
+<Solution>
+  <Configurations>
+    <Platform Name="Any CPU" />
+    <Platform Name="x64" />
+    <Platform Name="x86" />
+  </Configurations>
+  <Folder Name="/src/">
+    <Project Path="src/GitHub.Copilot.SDK.csproj" />
+  </Folder>
+  <Folder Name="/test/">
+    <Project Path="test/GitHub.Copilot.SDK.Test.csproj" />
+  </Folder>
+</Solution>