Fix our IsExternalInit workaround

We were also missing the conditional type forward.

Author: andyleejordan

src/PowerShellEditorServices/IsExternalInit.cs

@@ -0,0 +1,19 @@
+#pragma warning disable IDE0073
+#if NET5_0_OR_GREATER
+[assembly: System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Runtime.CompilerServices.IsExternalInit))]
+#else
+
+using System.ComponentModel;
+
+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