Allow C# 9 records and init-only properties by sandboxing.

The System.Runtime.CompilerServices.IsExternalInit type needed to be added since it is used as a modreq.

Robust.Shared/ContentPack/Sandbox.yml

@@ -433,6 +433,7 @@ Types:
     ExtensionAttribute: { All: True }
     IAsyncStateMachine: { All: True }
     InternalsVisibleToAttribute: { All: True }
+    IsExternalInit: { All: True }
     IsReadOnlyAttribute: { All: True }
     IteratorStateMachineAttribute: { All: True }
     RuntimeCompatibilityAttribute: { All: True }