Remove ObservableObject base from relay commands

[Sergio0694]

Describe the problem this feature would solve

Right now both asynchronous command types inherit from ObservableObject.
This has a number of issues:

• This also gives the type support for INotifyPropertyChanging, which is not applicable here.
• This prevents us from being able to add a number of optimizations in the internal properties.

Describe the solution

The following changes need to be done to the two asynchronous command types:

 namespace CommunityToolkit.Mvvm.Input
 {
     public sealed class AsyncRelayCommand :
-        ObservableObject, 
         IAsyncRelayCommand
     {
+        public event PropertyChangedEventHandler? PropertyChanged;
     }
 
     public sealed class AsyncRelayCommand<T> :
-        ObservableObject,
         IAsyncRelayCommand<T>
     {
+        public event PropertyChangedEventHandler? PropertyChanged;
     }
 }

Along with this, some specific optimizations and refactorings can also be applied on their implementation.

Describe alternatives you've considered

We could leave them as they are, but that'd be suboptimal. This would likely not be source breaking for almost everyone.
Still leaving the breaking change label as this could technically be a binary breaking change.