`UnreachableException` polyfill

[alexrp]

• https://learn.microsoft.com/en-us/dotnet/api/system.diagnostics.unreachableexception?view=net-7.0
• https://github.com/dotnet/runtime/blob/main/src/libraries/System.Private.CoreLib/src/System/Diagnostics/UnreachableException.cs

This exception is not really intended to be caught as it being thrown indicates that something has gone horribly wrong, like a Debug.Assert() failing or Environment.FailFast() being called.

I have several code paths in some of my analyzer and source generator projects where throwing UnreachableException is the obvious thing to do, but the type is net7.0 only.

[dnperfors]

I have a separate package for this, but I am willing to include it in pollysharp.. (https://github.com/dnperfors/unreachableexception/)