feat: add scoped custom elements to FASTElement

[janechu]

🙋 Feature Request

The scoped custom elements are now in Safari Tech Preview and are due to be rolled out in other browsers. They should be supported in FASTElement going forward.

💁 Possible Solution

const registry = new CustomElementRegistry;

FASTElement.define({
    name: "my-element",
    shadowOptions: {
        customElements: registry
    }
});

🔦 Context

whatwg/html#10854