IRelayCommand.Executed and IAsyncRelayCommand.Executed events

### Overview

Allows the user to respond to the execution of commands in a reactive manner. This is a fairly common pattern, especially for people who have used reactive code before.

### API breakdown

```cs
namespace CommunityToolkit.Mvvm.Input;

public interface IRelayCommand : ICommand
{
public event EventHandler Executed;
}
```

### Usage example

Reactive code, some checks after command execution

### Breaking change?

No

### Alternatives

ReactiveUI.ReactiveCommand

### Additional context

_No response_

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