Span2D Constructor overload

### Overview

A constructor for the `Span2D` to create a 2D view over a `Span`. Currently only arrays and pointers but I believe a `Span` would also be suitable. In fact, I think I can see an implementation already there, it's just marked as internal.

### API breakdown

```cs
public readonly ref partial struct Span2D
{
// ...
public Span2D(Span span, int height, int width) : this (span, 0, height, width, 0) {...}

public Span2D(Span span, int offset, int height, int width, int pitch) {...}
// ...
}
```

### Usage example

```cs
Span p = stackalloc int[9];
Span2D span = new Span2D(p, 3, 3);
```

### Breaking change?

No

### Alternatives

```cs
Span span = stackalloc int[6];
Span2D span2d = Span2D.DangerousCreate(ref span[0], 2, 3, 0);
```

```cs
int* p = stackalloc int[6];
Span2D span2d = new Span2D(p, 2, 3, 0);
```

### Additional context

_No response_

### Help us help you

Yes, but only if others can assist