Add "polyfill" for IsExternalInit C# feature

Imperium does this as well, and the whole internet would tell you to do
this too, so it should be fine.

MuzikaGromche/ExternalInit.cs

@@ -0,0 +1,6 @@
+// ReSharper disable once CheckNamespace
+#pragma warning disable IDE0130 // Namespace does not match folder structure
+namespace System.Runtime.CompilerServices
+{
+    internal static class IsExternalInit;
+}