[whatwg/webidl] Fix "inclusive inherited interfaces" to walk the inheritance chain (PR #1658)

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The [inherited interfaces](https://webidl.spec.whatwg.org/#dfn-inherited-interfaces) of an interface are defined recursively: the interface it directly inherits from, plus all of *that* interface's inherited interfaces. So [inclusive inherited interfaces](https://webidl.spec.whatwg.org/#interface-inclusive-inherited-interfaces) should walk the whole inheritance chain.

But step 3.2 advances from the fixed input |I| instead of the loop variable |interface|:

> Set |interface| to the interface that |I| inherits from, if any, and null otherwise.

So after the first iteration, |interface| is always |I|'s direct parent and never becomes null. The loop never ends for any interface that inherits from another. This PR changes |I| to |interface| so each iteration moves one level up the chain.

The algorithm is used by:
- [implements](https://webidl.spec.whatwg.org/#implements), which regular operations and attributes use to check their `this` value
- [internally create a new object implementing the interface](https://webidl.spec.whatwg.org/#internally-create-a-new-object-implementing-the-interface), which copies each ancestor's `[LegacyUnforgeable]` members onto the new instance.

For example, `CustomEvent` inherits from `Event`, which has a `[LegacyUnforgeable]` attribute:

```webidl
interface CustomEvent : Event { ... };

interface Event {
  ...
  [LegacyUnforgeable] readonly attribute boolean isTrusted;
  ...
};
```

Under the current text, `new CustomEvent("foo")` would never finish. With the fix, it returns an object with `isTrusted` as an own property whose value is `false`.

Implementations seems to behave this way. Chromium, Gecko, and WebKit pass dom/idlharness.any.js, which includes test for `new CustomEvent("foo")`, on wpt.fyi.

This PR only aligns the spec with existing behavior, so it needs no new tests and no implementation bugs.

- [ ] At least two implementers are interested (and none opposed):
   * …
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- [ ] [Implementation bugs](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) are filed:
   * Chromium: …
   * Gecko: …
   * WebKit: …
   * Deno: …
   * Node.js: …
   * webidl2.js: …
   * widlparser: …
- [ ] [MDN issue](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) is filed: …
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You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/webidl/pull/1658

-- Commit Summary --

  * Fix "inclusive inherited interfaces" to walk the inheritance chain

-- File Changes --

    M index.bs (4)

-- Patch Links --

https://github.com/whatwg/webidl/pull/1658.patch
https://github.com/whatwg/webidl/pull/1658.diff

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