In scripthost.d.ts add `WScript` interface for extendibility

lib Update Request

I want to write a type lib to extend WScript, allow CreateObject to recognize some specified ProgId, and make the returned COMObject typed:

var app = WScript.CreateObject("Microsoft.Excel");
//   ^^ 
//   make this typed

But in scripthost.d.ts, The type of WScript is inlined with it's declearation:

declare var WScript: { /* inlined type definition */ };

To make it possible to extend, we can add a interface using the same name WScript and set this as the type of the var WScript:

interface WScript { /* type definition of WScript */ }

declare var WScript: WScript;

In thiry part lib, they could extend WScript var by extends the interface WScript. for example, In my lib, I could write:

interface WScript {
    CreateObject("Microsoft.Excel"): Excel.App;
}

declare namespace Excel {
    interface App {
        //...
    }
}

Inizia in scripthost.d.ts individuando la dichiarazione inline di WScript e la firma di CreateObject. Refattorizza la dichiarazione in modo che WScript sia un'interfaccia utilizzata dalla variabile globale e verifica che un'estensione dell'interfaccia di terze parti possa specializzare CreateObject per un ProgId specificato e restituire un oggetto COM tipizzato.