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

**TypeScript Version:** 2.5.0 / nightly (2.6.0-dev.20170902) **Code** ```ts // 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: ```typescript // Compiler error x = { Workbooks: []; }; ``` and also prevent using `new` to create a new instance of the inheriting type: ```typescript // Compiler error x = new Excel.Application(); ``` --- I've [opened an issue](https://github.com/Microsoft/TypeScript/issues/17526) 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: ```typescript declare class VarDate extends AutomationObject<VarDate> { } declare class SafeArray<T> extends AutomationObject<SafeArray<T>> { } ```