Expose LocalSerializer

**Use case:** I want to be able to deserialize a `Pulumi.stack.yml` file without using a `LocalWorkspace`, as that requires too much configuration and isn't very well-documented.

sdk/Pulumi.Automation/Serialization/LocalSerializer.cs

@@ -10,7 +10,7 @@
 
 namespace Pulumi.Automation.Serialization
 {
-    internal class LocalSerializer
+    public class LocalSerializer
     {
         private readonly JsonSerializerOptions _jsonOptions;
         private readonly IDeserializer _yamlDeserializer;