[heycam/webidl] Proposal: allow subclasses of constructible built-ins to be constructed (#125)

Originally: https://github.com/whatwg/html/issues/1289

### The problem

`HTMLElement` has a constructor, specifically designed so that you can do

```js
class X extends HTMLElement {}
customElements.define("x-x", X);
new X();
```

However, our intention was that you should also be able to do

```js
class Y extends HTMLParagraphElement {}
customElements.define("y-y", Y, { extends: "p" });
new Y();
```

But this currently fails since, per Web IDL, the first step of `HTMLParagraphElement`'s constructor is to throw a `TypeError`.

### Proposed solution

Change the first step of all Web IDL interface object's construction behavior to only throw if they are not a derived interface. Otherwise, do the equivalent of `super(...arguments)`.

This means all subclasses of things that are constructible automatically get a pass-through constructor.

Subclasses of things without constructors also get a pass-through constructor, but since the superclass constructor throws, the subclass constructor will continue throwing.

As part of this, I'd update the specification for interface objects to have modern [[Construct]] and [[Call]] definitions instead of just [[Call]].

### Repercussions

I'm not sure if there are any interface on the platform that currently have a constructor, but which have subclasses that are not constructible. If such interfaces exist, their subclasses would become constructible. This seems unlikely to be a problem even if such interfaces do exist, but it's hard to say for sure.

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I'm happy to do a PR for this but would like some feedback on whether it will be accepted.

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