Update ImmutabilityContext.Factory.cs (#965)

Add SemaphoreSlim to default types.

https://learn.microsoft.com/en-us/dotnet/api/system.threading.semaphoreslim?view=netframework-4.8

From this PR Comment

Author: sgrinwis-d2l

src/D2L.CodeStyle.Analyzers/Immutability/ImmutabilityContext.Factory.cs

@@ -54,6 +54,7 @@ internal partial class ImmutabilityContext {
 			("System.Text.RegularExpressions.Regex", default),
 			("System.Text.UTF8Encoding", default),
 			("System.Threading.ReaderWriterLockSlim", default),
+			("System.Threading.SemaphoreSlim", default),
 			("System.TimeSpan", default),
 			("System.Type", default),
 			("System.Uri", default),