Proposal: File scoped namespaces

Spec: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-10.0/file-scoped-namespaces.md

How it looks now:
```
using System;

namespace Company.Project
{
public class Product
{
...
}
}
```

Why do we need to nest our class definition in namespace ? Why not to remove extra nesting ?
Isn't that better ?

```
namespace Company.Project
using System;

public class Product
{
...
}
```

LDM history:
- https://github.com/dotnet/csharplang/blob/main/meetings/2021/LDM-2021-02-10.md#namespace-directives-in-top-level-programs
- https://github.com/dotnet/csharplang/blob/main/meetings/2021/LDM-2021-01-13.md#file-scoped-namespaces
- https://github.com/dotnet/csharplang/blob/main/meetings/2021/LDM-2021-01-05.md#file-scoped-namespaces