Add warning message if using the .NET Standard platform. It was previous happening before this version came in this file https://github.com/reactiveui/ReactiveUI/blob/08503a73818157705765a0fa6742718ec4de65dd/src/ReactiveUI/Platforms/netstandard2.0/PlatformRegistrations.cs#L20 We have to be careful so we don't confuse WASM users as well.