AsyncRelayCommand IsRunning only true on return of Task

### Describe the bug

The ExecutionTask and IsRunning of the AsyncRelayCommand is only set after the execute returns a Task, so on the first real async function.
As long as it's running synchronously, the ExecutionTask is still the previous task and IsRunning is based on that one

### Regression

_No response_

### Steps to reproduce

1. Create an AsyncRelayCommand with an async function that returns a Task
```
relayCommand = AsyncRelayCommand(Execute);
```
2. Check if is IsRunning in the synchronous part of the execution method
```
async Task Execute()
{
Debug.Assert(relayCommand.IsRunnung); // Fails
Debug.Assert(relayCommand.AsyncRelayCommand == null); // at least not be the previous task?
await Task.Wait(1000);
}
```

### Expected behavior

That AsyncRelayCommand is set to null and IsRunning is true before the execute method is called.

### Screenshots

_No response_

### IDE and version

VS 2022

### IDE version

_No response_

### Nuget packages

- [ ] CommunityToolkit.Common
- [ ] CommunityToolkit.Diagnostics
- [ ] CommunityToolkit.HighPerformance
- [x] CommunityToolkit.Mvvm (aka MVVM Toolkit)

### Nuget package version(s)

8.2.0

### Additional context

Ideally, you'd not call the IsRunning inside the Execute call, but I've done it by accident already that I run setup that I wanted to skip when a command is running, that is called from the Execute function before any network calls.

Workaround is just doing
```
await Task.Yield();
```
as the first line in the Execute method.

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