Support the ability to emit certain types as interfaces with construct signatures (instead of classes)

[jumpinjackie]

Currently if you take a random Cesium sandcastle example (for example):

// Add several billboards based on the above image in the atlas.
    viewer.entities.add({
        position : Cesium.Cartesian3.fromDegrees(-75.59777, 40.03883),
        billboard : {
            image : '../images/whiteShapes.png',
            imageSubRegion : new Cesium.BoundingRectangle(49, 43, 18, 18),
            color : Cesium.Color.LIME
        }
    });
    viewer.entities.add({
        position : Cesium.Cartesian3.fromDegrees(-84.0, 39.0),
        billboard : {
            image : '../images/whiteShapes.png',
            imageSubRegion : new Cesium.BoundingRectangle(61, 23, 18, 18),
            color : new Cesium.Color(0, 0.5, 1.0, 1.0)
        }
    });

It is compiler errors galore on the objects being added. This is because the API documentation states that these are meant to be Cesium.Entity instances. But because Cesium.Entity is a class, it is not assignable from an object that happens to have the same "shape" as it.

From this I strongly suspect that Entity should really be a TS interface, but have a declared var of the same name that defines the constructor.

So where we currently have this

class Entity {
    constructor(...);
    id: string;
    name: string;
    ...
    addProperty(propertyName: string);
}

The plugin should have the ability for us to say that Entity should be an interface, that should then generate something like this:

interface Entity {
    id: string;
    name: string;
    ...
    addProperty(propertyName: string);
}
declare const Entity: {
    new (...): Entity
}

With this pattern (and a switch that allows all members of the class-now-interface to be optional), it should allow for the above sandcastle example to be legal.

This is how TypeScript defines built-ins like Object, String, etc.