Make the Optional keyword optional

_just porting my old vb.net suggestions from uservoice_

Please allow
```
Private Sub Test(s As String = 1)
```
as in C# besides the current version:
```
Private Sub Test(Optional s As String = 1)
```
This removes redundancies (--> improves readability, accelerates writing code) and drives forward the co-evolution of VB/C#. I don't see any reason not to allow this.