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 {
        //...
    }
}

Comienza en scripthost.d.ts localizando la declaración inline de WScript y la firma de CreateObject. Refactoriza la declaración para que WScript sea una interfaz utilizada por la variable global y confirma que una ampliación de interfaz de terceros puede especializar CreateObject para un ProgId especificado y devolver un objeto COM tipado.