Support Web Components

[gterzian]

As per MDN, Web Components is "is a suite of different technologies allowing you to create reusable custom elements". This issue is a place to track our support for these technologies.

• [ ] [CustomElementRegistry](https://html.spec.whatwg.org/multipage/custom-elements.html#customelementregistry). We have the interface, the status of its capabilities is unknown.
• [ ] Shadow DOM: tracked at https://github.com/servo/servo/issues/34318 (and includes some of the below)
• [ ] HTML templates elements: we have the interface, the status of its capabilities is unknown.
• [ ] HTMLSlotElement: doesn't seem to be supported yet.
• [ ] Custom elements: I don't know.

[jdm]

We implemented custom elements and the registry as a gaoc project. Lots of tests are passing but there's still room for improvement: https://wpt.fyi/results/custom-elements?label=master&label=experimental&product=chrome&product=firefox&product=servo&aligned

https://github.com/servo/servo/issues/9372