Add Declarative Structured Concurrency and Scheduling

[timonkrebs]

• https://vorpus.org/blog/notes-on-structured-concurrency-or-go-statement-considered-harmful/

• https://github.com/StephenCleary/StructuredConcurrency

• https://github.com/apple/swift-evolution/blob/main/proposals/0304-structured-concurrency.md

• https://github.com/apple/swift-evolution/blob/main/proposals/0306-actors.md

• https://github.com/apple/swift-evolution/blob/main/proposals/0313-actor-isolation-control.md

• https://github.com/apple/swift-evolution/blob/main/proposals/0336-distributed-actor-isolation.md

• https://reactivex.io/documentation/scheduler.html

• https://github.com/dotnet/reactive/blob/main/Rx.NET/Source/src/System.Reactive/Concurrency/TaskPoolScheduler.cs

• https://stackoverflow.com/questions/4331262/task-continuation-on-ui-thread/4331287#4331287

• https://learn.microsoft.com/en-us/dotnet/api/system.threading.tasks.task.continuewith?view=net-7.0&redirectedfrom=MSDN#System_Threading_Tasks_Task_ContinueWith_System_Action_System_Threading_Tasks_Task__System_Threading_Tasks_TaskContinuationOptions_