Add base class in lib.scripthost.d.ts for Automation objects

TypeScript Version: 2.5.0 / nightly (2.6.0-dev.20170902)

Code

// in lib.scripthost.d.ts
class AutomationObject<T> {
  private constructor();
  private typekey: AutomationObject<T>;
}

// in activex-excel.d.ts
declare namespace Excel {
    class Application extends AutomationObject<Application> {
        Workbooks: any;
    }
}

interface ActiveXObject {
    new(progid: 'Excel.Application'): Excel.Application;
}
    
// usage
let x: Excel.Application = new ActiveXObject('Excel.Application');

This currently wouldn't compile, because of the private constructor in the base class; but pending resolution of #18283, it would be possible.

This would prevent assigning structurally matching objects to the inheriting types:

// Compiler error
x = {
    Workbooks: [];
};

and also prevent using new to create a new instance of the inheriting type:

// Compiler error
x = new Excel.Application();

I've opened an issue to define similar behavior for VarDate and SafeArray<T> in lib.scripthost.d.ts. If AutomationObject<T> is defined, then it could be used in lib.scripthost.d.ts as a base class for these types as well:

declare class VarDate extends AutomationObject<VarDate> { }
declare class SafeArray<T> extends AutomationObject<SafeArray<T>> { }

Start with lib.scripthost.d.ts and review the proposed AutomationObject declaration and its use as a base for VarDate and SafeArray. Check the resolution of issues #18283 and #17526 first; done means the declarations enforce the described structural and construction restrictions without breaking the scripthost definitions.