Add support for WebIDL mixins

[tobie]

Hi all!

WebIDL recently introduced dedicated syntax for mixins.

This syntax replaces the [NoInterfaceObject] extended attribute and implements statement which have been deprecated (except for a few legacy uses cases explicitely mentioned in the spec).

You can read more about it here.

In most cases, the changes should be relatively straightforward. The now deprecated:

interface Foo { };

[NoInterfaceObject]  // (Mostly) DEPRECATED
interface Bar { };
Foo implementes Bar; // DEPRECATED

should just be rewritten as:

interface Foo { };

interface mixin Bar { };
Foo includes Bar;

We're filing issues against specs which use the deprecated syntax, but we welcome help to warn editors about these changes.

We also filed an issue against WebIDL2 here: w3c/webidl2.js#112.

Please feel free to reach out if you have any questions.

Thanks!

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Tracked in: whatwg/webidl#472 | Original pull-request: whatwg/webidl#433

[marcoscaceres]

Have it implemented locally :)