Added solution filter for .NET projects

Windows Community Toolkit (NET).slnf

@@ -0,0 +1,15 @@
+{
+  "solution": {
+    "path": "Windows Community Toolkit.sln",
+    "projects": [
+      "Microsoft.Toolkit.Diagnostics\\Microsoft.Toolkit.Diagnostics.csproj",
+      "Microsoft.Toolkit.HighPerformance\\Microsoft.Toolkit.HighPerformance.csproj",
+      "Microsoft.Toolkit.Mvvm\\Microsoft.Toolkit.Mvvm.csproj",
+      "Microsoft.Toolkit\\Microsoft.Toolkit.csproj",
+      "UnitTests\\UnitTests.HighPerformance.NetCore\\UnitTests.HighPerformance.NetCore.csproj",
+      "UnitTests\\UnitTests.HighPerformance.Shared\\UnitTests.HighPerformance.Shared.shproj",
+      "UnitTests\\UnitTests.NetCore\\UnitTests.NetCore.csproj",
+      "UnitTests\\UnitTests.Shared\\UnitTests.Shared.shproj"
+    ]
+  }
+}