Provide a way to specify inheritance explicitly

**TypeScript Version:** 3.0.0-dev.201xxxxx Its seems there's really no way currently to setup inheritance when working with complex mixins, and/or extending prototypes manually. While mixin support landed on TypeScript recently, it only addresses very simple cases. It's usefulness ends when you also want mixin that inherit from different classes on their own. To illustrate it with an example: ```js export class Component extends HTMLElement implements ComponentCore { constructor() { super(); ComponentCore.init(this); } } ``` where the ComponentCore.init does mixin style implementation of ComponentCore. Tried the mixin approach documented, but that doesn't seem to work when you want to extend from HTMLElement here. Now, moving on a real-life example with a lot more context: https://github.com/prasannavl/icomponent/blob/v5.1.1/packages/icomponent/src/component.ts#L6 I had to jump through hoops and do something like ```ts ComponentCore.extend(ComponentImpl); export interface IComponent extends ComponentImpl, ComponentStatics, ComponentCore, Constructor<IComponent> { } export const Component: IComponent = ComponentImpl as any; ``` in order to even get inheritance working properly. This is further complicated, when you extend from these classes, since there's currently it's ` const Component`, and as such it is not always interpreted by the compiler as a class -- As as example - a few lines down in the same: https://github.com/prasannavl/icomponent/blob/v5.1.1/packages/icomponent/src/component.ts#L30 You can't pass `Component` into generics parameters, since it's no longer a class. **Possible solutions**: - A simple and effective solution would be to just be even annotate the class and having different parents. // @ts-inherit: IComponentCore, HTMLElement, etc, - Extend the `extends` syntax to inherit multiple classes with say "extends X, !Y, !Z` -- The exclamation is the indicator of these special classes - Allow implements to inherit a clas…