feat: update .editorconfig to enforce severity for nullable reference type diagnostics

Author: https-richardy

.editorconfig

@@ -18,15 +18,16 @@ dotnet_diagnostic.IDE2000.severity = error
 dotnet_diagnostic.IDE2001.severity = error
 dotnet_diagnostic.IDE2002.severity = error
 
-# unnecessary usings (https://learn.microsoft.com/en-us/dotnet/fundamentals/code-analysis/style-rules/ide0005)
 dotnet_diagnostic.IDE0005.severity = error
-
-# unused variables (https://learn.microsoft.com/en-us/dotnet/csharp/misc/cs0168)
 dotnet_diagnostic.CS0168.severity = error
-
-# assigned but never used (https://learn.microsoft.com/en-us/dotnet/csharp/misc/cs0219)
 dotnet_diagnostic.CS0219.severity = error
 
+dotnet_diagnostic.CS8600.severity = error
+dotnet_diagnostic.CS8601.severity = error
+dotnet_diagnostic.CS8602.severity = error
+dotnet_diagnostic.CS8603.severity = error
+dotnet_diagnostic.CS8604.severity = error
+
 [*.json]
 indent_style = space
 indent_size = 4