Update the template screen to use the new(ish) Observation framework. (#4077)

* Update the template screen to use the new(ish) Observation framework.

* Add a variant of deferFulfillment that supports observables.

* Update snapshot fulfilment to work with either a publisher or a stream.

Tools/Scripts/Templates/SimpleScreenExample/ElementX/TemplateScreenModels.swift

@@ -16,6 +16,8 @@ enum TemplateScreenViewModelAction {
 struct TemplateScreenViewState: BindableState {
     var title: String
     var placeholder: String
+    var counter = 0
+    
     var bindings: TemplateScreenViewStateBindings
 }
 
@@ -27,5 +29,8 @@ enum TemplateScreenViewAction {
     case done
     case textChanged
     
+    case incrementCounter
+    case decrementCounter
+    
     // Consider adding CustomStringConvertible conformance if the actions contain PII
 }