Enable other classes that already implement the `INotifyPropertyChanged` to leverage Source Generator Attributes

### Overview

For instance, I already have a class I'm using which inherits from `INotifyPropertyChanged`. In my scenario, it's `ObservableCollection`, so I can't modify it as it's in the BCL.

```cs
public class ObservableCollection : Collection, INotifyCollectionChanged, INotifyPropertyChanged
```

It already has an `OnPropertyChanged` helper and `PropertyChanged` event:

```cs
protected virtual event PropertyChangedEventHandler? PropertyChanged;

protected virtual void OnPropertyChanged(PropertyChangedEventArgs e)
{
PropertyChanged?.Invoke(this, e);
}
```

It would be great if I could still use some or all of the MVVM attributes for source generators like `[ObservableProperty]` when inheriting from these classes in my own class.

### API breakdown

N/A uses existing attributes.

### Usage example

For instance, I have a class already inheriting from this:

```cs
public partial class MyClass : ObservableCollection
{
// Since my parent class already inherits from INotifyPropertyChanged (and already has a OnPropertyChanged method), just use those...
[ObservableProperty]
private bool _isModified;
}
```

Worst case, let me have to re-specify that I want whatever's needed to polyfill, either with `INotifyPropertyChanged` or `ObservableObject`:

```cs
[INotifyPropertyChanged]
public partial class MyClass : ObservableCollection
{
[ObservableProperty]
private bool _isModified;

partial OnIsModifiedChanged(bool value)
{
// Yay, I could also do this now...
}
}
```

Basically the existing generator in this case would spit out all the same stuff except what's already implemented by the type (in the case of `ObservableCollection` it'd be the following:

```cs
partial class MyClass : global::System.ComponentModel.INotifyPropertyChanged
{
//// The Event and the event args OnPropertyChanged are already implemented, so skip adding them

///
/// Raises the event.
///
/// (optional) The name of the property that changed.
[global::System.CodeDom.Compiler.GeneratedCode("CommunityToolkit.Mvvm.SourceGenerators.INotifyPropertyChangedGenerator", "8.1.0.0")]
[global::System.Diagnostics.DebuggerNonUserCode]
[global::System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverage]
protected void OnPropertyChanged([global::System.Runtime.CompilerServices.CallerMemberName] string? propertyName = null)
{
OnPropertyChanged(new global::System.ComponentModel.PropertyChangedEventArgs(propertyName));
}

//// All the SetProperty helpers, TaskNotifier, etc...
```

If there's any conflict for some particular type, then raise an analytic warning/error.

### Breaking change?

No

### Alternatives

Can't use MVVM Toolkit generators or helpers? 😥

### Additional context

Don't know if I know enough to help here. Had a hard time trying to understand how the existing code was generated with the new fanciness...

### Help us help you

Yes, if someone can help (I'm not sure I understand where the current `INotifyPropertyChanged` generator is and how it works. Nor do I know if I know how to detect if something inherits or has the required methods to skip outputting them... but beyond that, I'd be willing to assist giving it a go if pointed in the right directions.