chore: support extension through abstract base class

[eladb]

This is a proposal based on this comment.

The basic idea is that both the polycon class (e.g. Dog) and the concrete class (Poodle) extend a common abstract base type (e.g. DogBase).

This has the following effects:

1. No need to protect against stack overflow (the concrete class doesn't actually extend the polycon).
2. It is now possible to instantiate any concrete type directly (added a test that instantiates a Labrador even if when Dog resolves to Poodle.
3. We can still share common implementation at the base.
4. Enforce that concrete classes actually implement all abstract methods and properties (previously this was only by convention).

The polycon constructor has a quirky pattern:

constructor(scope, id, props) {
  super(null as any, id, props);
  return Polycons.create(QUALIFIER, scope, id, props);
}

The call to super() with null as the scope does two things:

1. The base construct is not attached to our tree.
2. The null can be used by the base class to bail out early:

class DogBase {
  public readonly foo: number;

  constructor(scope, id, props) {
    super(scope, id);
  
    if (!scope) {
      // initialize all readonly props with dummy values, this obj is thrown away
      this.foo = -1;
      return;
    }

    this.foo = 7947; // <-- the real thing
  }
}