Add template for IsExternalInit file (#2)

content/IsExternalInit/.template.config/template.json

@@ -0,0 +1,15 @@
+{
+    "$schema": "http://json.schemastore.org/template",
+    "author": "Insight Architectures",
+    "classifications": ["IA", "IsExternalInit"],
+    "name": "[IA] IsExternalInit",
+    "identity": "InsightArchitectures.Templates.Items.IsExternalInit",
+    "groupIdentity": "InsightArchitectures.Templates.Items.IsExternalInit.CSharp",
+    "shortName": "isexternalinit",
+    "tags": {
+        "language": "C#",
+        "type": "item"
+    },
+    "defaultName": "IsExternalInit",
+    "description": "This templates creates a IsExternalInit file that adds low-level support for init-only properties in runtimes prior to .NET 5.0."
+}

content/IsExternalInit/IsExternalInit.cs

@@ -0,0 +1,26 @@
+//-:cnd:noEmit
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+// See: https://github.com/dotnet/roslyn/issues/45510
+
+#if NETSTANDARD2_0 || NETSTANDARD2_1 || NETCOREAPP2_0 || NETCOREAPP2_1 || NETCOREAPP2_2 || NETCOREAPP3_0 || NETCOREAPP3_1 || NET45 || NET451 || NET452 || NET6 || NET461 || NET462 || NET47 || NET471 || NET472 || NET48
+
+using System.ComponentModel;
+
+// ReSharper disable once CheckNamespace
+namespace System.Runtime.CompilerServices
+{
+    /// <summary>
+    /// Reserved to be used by the compiler for tracking metadata.
+    /// This class should not be used by developers in source code.
+    /// </summary>
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    internal static class IsExternalInit
+    {
+    }
+}
+
+#endif
+//+:cnd:noEmit