This PR is a work in progress to solicit feedback on the proposed solution, please do not merge or use this branch.

This commit attempts to make progress towards #5863. It is not a comprehensive solution, but it does provide a way for a library writer to 'opt-in' and provide an interface with a static polymorphic this.

One of the difficulties with #5863, is how to make it clear which this the this type refers to, and the issue of the static and instance type mixing in the same declaration block.

This sidesteps those issues, by only providing a static polymorphic this when using an interface in the extends position. This makes the meaning of this very clear and it's explicit which side the this type falls on; there is no mixing.
i.e:

interface InstType {
  member: this;    // instance this
}

interface CtorType {
  new(): InstType;
  poly_prop: this; // static this
}

declare var X: CtorType;
class Derived extends X {
  ex1() {}
  static ex2() {}
}

const inst = new Derived();
inst.member.ex1();         // instance polymorphic
Derived.poly_prop.ex2();   // static polymorphic

WIP

The PR is not finished. There are a few situations that need to be resolved and tests need to be written if it's decided that this is an acceptable solution.

(Potentially) fixes #5863