Replace `CompletionDelegate` with `Action<CompletionContext>`

```csharp
namespace System.CommandLine.Completions
{
    // This is syntactically the same as Action<CompletionContext>, so adding a new delegate type is discouraged.
    // Unless you think you need the type to exists as a place to put documentation.
    public delegate IEnumerable<CompletionItem> CompletionDelegate(CompletionContext context);
```