Add section on using Scala 3 Union types to eliminate msg adapters

- Use InteractionPatternsSpec as an example for applying Scala 3's
  Union types to symplify actor code
  - Remove response message wrapper & adapter
  - Use the union of the actor's public protocol (the `Translate`
    message (the only member of the `Command` ADT) and the possible
    responses from the backend (messages `JobStarted`, `JobProgress`,
    and `JobCompleted`): `private type CommandAndResponse = Command | Backend.Response`
  - Instead of utilising the message adaptor `ActorRef` in the `replyTo`
    field of the message sent to the backend, `context.self` is used
    instead
  - The internal (extended) `Behavior[CommandAndResponse]` is narrowed
    to `Behavior[Command]` at creation time

The diffs between the original and the new version:

```bash
136d139
<         private final case class WrappedBackendResponse(response: Backend.Response) extends Command
137a141,142
>         private type CommandAndResponse = Command | Backend.Response
>
139,141c144
<           Behaviors.setup[Command] { context =>
<             val backendResponseMapper: ActorRef[Backend.Response] =
<               context.messageAdapter(rsp => WrappedBackendResponse(rsp))
---
>           Behaviors.setup[CommandAndResponse] { context =>
143,144c146,147
<             def active(inProgress: Map[Int, ActorRef[URI]], count: Int): Behavior[Command] = {
<               Behaviors.receiveMessage[Command] {
---
>             def active(inProgress: Map[Int, ActorRef[URI]], count: Int): Behavior[CommandAndResponse] = {
>               Behaviors.receiveMessage[CommandAndResponse] {
147c150
<                   backend ! Backend.StartTranslationJob(taskId, site, backendResponseMapper)
---
>                   backend ! Backend.StartTranslationJob(taskId, site, context.self)
150,162c153,162
<                 case wrapped: WrappedBackendResponse =>
<                   wrapped.response match {
<                     case Backend.JobStarted(taskId) =>
<                       context.log.info("Started {}", taskId)
<                       Behaviors.same
<                     case Backend.JobProgress(taskId, progress) =>
<                       context.log.info2("Progress {}: {}", taskId, progress)
<                       Behaviors.same
<                     case Backend.JobCompleted(taskId, result) =>
<                       context.log.info2("Completed {}: {}", taskId, result)
<                       inProgress(taskId) ! result
<                       active(inProgress - taskId, count)
<                   }
---
>                 case Backend.JobStarted(taskId) =>
>                   context.log.info("Started {}", taskId)
>                   Behaviors.same
>                 case Backend.JobProgress(taskId, progress) =>
>                   context.log.info2("Progress {}: {}", taskId, progress)
>                   Behaviors.same
>                 case Backend.JobCompleted(taskId, result) =>
>                   context.log.info2("Completed {}: {}", taskId, result)
>                   inProgress(taskId) ! result
>                   active(inProgress - taskId, count)
167c167
<           }
---
>           }.narrow
```