.Net: Added shared classes (#58)

* Added Throw shared class

* Added Experimental attribute

* Added IsExternalInit

* Added TrimAttributes

* Small update

* Small update

* Suppress warning

* Added comment

* Updated LegacySupport.props

* Small updates

dotnet/src/LegacySupport/IsExternalInit/IsExternalInit.cs

@@ -0,0 +1,9 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+/* This enables support for C# 9/10 records on older frameworks */
+
+namespace System.Runtime.CompilerServices;
+
+internal static class IsExternalInit
+{
+}

dotnet/src/LegacySupport/IsExternalInit/README.md

@@ -0,0 +1,11 @@
+# IsExternalInit
+
+Enables use of C# record types on older frameworks.
+
+To use this source in your project, add the following to your `.csproj` file:
+
+```xml
+<PropertyGroup>
+  <InjectIsExternalInitOnLegacy>true</InjectIsExternalInitOnLegacy>
+</PropertyGroup>
+```