Add backwards compatibility for ObservableGroupedCollection extensions.

### Overview

Whilst upgrading from `CommunityToolkit.Common.Collections` to `CommunityToolkit.Mvvm`, I noticed that a lot of (linq e.a.) extensions on ObservableGroupedCollection and derived classes I am using in my code are no longer available.

For example;

collection.OrderBy
collection.Sum
collection.FirstOrDefault
collection.Select
collection.SelectMany

It appears as if a previously implemented interface (IEnumerable?) is no longer implemented.

Can this be added 'back' please? I can't find any other viable workarounds.

### API breakdown

_I'm not able to provide an API breakdown._

### Usage example

```csharp
var all = this._groupedRegistrations.SelectMany(x => x).ToList();
var location = this._groupedRegistrations.Select(x => x.Key).Append(groupCharacter).OrderBy(x => x).TakeWhile(x => x != groupCharacter).Count();
var cnt = this.GroupedRegistrations.Sum(g => g.Count);
```

### Breaking change?

No

### Alternatives

At this moment, I am using the old version of the CommunityToolkit as a workaround.

### Help us help you

No, just wanted to propose this