JS: Make getInstance() propagate to subclasses

[asgerf]

This makes the Instance MaD token obtain instances of the current class as well as instances of subclasses of the current class. For the moment this only works for sources (formerly known as use-nodes). I'll add a corresponding case for sinks (def-nodes) in a separate PR.

For example, foo;Member[C].Instance.Member[m] now matches the following:

import { C } from "foo";

new C().m(); // <-- found (same as before)

class D extends C {
  d() {
    this.m(); // <-- found (new)
}
new D().m(); // <-- found (new)

This aligns with how MaD for Ruby works, and resolves some issues that were raised in user feedback (see backlink).

[erik-krogh]

👍