Slice method in RefEnumerable<T> and ReadOnlyRefEnumerable<T>

### Overview

Slice method in RefEnumerable and ReadOnlyRefEnumerable analogue to Slice method of Span.

### API breakdown

```csharp
namespace CommunityToolkit.SomePackage;

public class ReadOnlyRefEnumerable
{
public ReadOnlyRefEnumerable Slice(int start,int length);
}
```

### Usage example

```csharp
var span=new Span2D(5,5);//matrix representation
span.GetColumn(0)[..3];//get 3 values from matrix

### Breaking change?

No

### Alternatives

Alternative is to copy everything to span.

### Additional context

_No response_

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