Prevent extenions/metaextension classes from being extended or used with instanceof

Classes decorated with @extension and @metaExtension should not be allowed to be extended or have instanceof called on them, since they are not TSTL-compiled classes.

```ts
declare class Foo {}

/** @extension */
declare class Bar extends Foo {}

class Baz extends Bar {} // No bueno

declare const obj: any;
if (obj instanceof Bar) {} // Also not a good idea
```
