Update Mixins page

The mixins page should be updated to reflect that we now support intersection classes.

[lostintime]

Also from version 2.7 with --strictPropertyInitialization option enabled mixed properties require definite assignment assertion modifiers to be used:

class SmartObject implements Disposable, Activatable {
    constructor() {
        setInterval(() => console.log(this.isActive + " : " + this.isDisposed), 500);
    }

    interact() {
        this.activate();
    }

    // Disposable
    isDisposed: boolean = false;
    dispose!: () => void;
    // Activatable
    isActive: boolean = false;
    activate!: () => void;
    deactivate!: () => void;
}

[dsherret]

There is also some information on this here that could be used to update this section.