Allow events to return values

C# supports it, vb works fine with c# events that returns values so I don't see any reason why vb events can't return values.

Syntax example:

```vb
Public Class SomeClass
Public Event Foo As Func(Of Integer, Task)
End Class

Public Class SomeOtherClass
Private WithEvents _someInstance As New SomeClass
Public Function OnFoo(x As Integer) As Task Handles _someInstance.Foo
Return Task.FromResult(x)
End Function
End Class
```