C# 14: Support for partial event declarations.

[michaelnebel]

In this PR we make support for partial event declarations.

That is, we support event declarations like.

public partial class Partial
{
    public partial event EventHandler MyEvent;
}

public partial class Partial
{
    public partial event EventHandler MyEvent {
        add { ... } 
        remove { ... } 
    }
}

The default add/remove event accessors from the declaring declaration are not extracted. Only the explicit add/remove from the implementation declaration are extracted. Note that a partial event always need to have an implementation declaration.