Clarify "Extending interfaces"

[dirkpostma]

In https://github.com/Microsoft/TypeScript-Handbook/blob/master/pages/Interfaces.md#extending-interfaces

The following syntax is used:

let square = <Square>{};

This is the first time the < > notation is used. I don't know what this is yet, so I don't have a usggestion to improve, except for saying: this should either be explained, or the example should be rewritten without this syntax.

[DanielRosenwasser]

That's a type assertion. It's a way of telling TypeScript that you should treat the {} as having the Square type. It's basically casting to another type with no runtime impact.

Also, duplicate of #123