Provide also raw interface definition instead of just class

[Granjow]

First, thanks for this helpful package @binier ! I used to manually declare onX(…) events to enforce typed events until I found it.

One thing I miss is a plain interface definition which I can inherit.

export interface ITypedEmitter<L extends ListenerSignature<L> = DefaultListener> {
    addListener<U extends keyof L>( event: U, listener: L[U] ): this;
    prependListener<U extends keyof L>( event: U, listener: L[U] ): this;
    // etc.
}

export interface MyInterfaceWithEvents extends ITypedEmitter<MyEvents> {
    // …
}

so users of the interface can also rely on typed events, not only users of the instance.