[FEATURE] Implement `Element`'s `getHTML()` to improve SSR of web components

Hello,

As per https://github.com/WebReflection/linkedom/issues/210#issuecomment-3803892846, I am filing an issue to add support for getHTML().

Recently baseline (2024) across browsers, makes sense to implement in Linkedom. MDN

Element.getHTML({serializableShadowRoots: true})

Example

<hello-world>
    <span>Slotted Hello World!</span>
</hello-world>

class HelloWorld extends HTMLElement {
    constructor() {
        super();
        this.attachShadow({ mode: "open", serializable: true });
    }

    ....
}

Rendered as this:

<hello-world>
      <span>Slotted Hello World</span>
      <template shadowroot="open" shadowrootserializable="">
          <style>
              :host {
                  display: inline-block;
                  background: pink;
                  border: 2px dashed yellow;
              }
          </style>
          <div><slot></slot></div>
      </template>
</hello-world>

The issue names no file or test. Start with the Element.getHTML API and the linked MDN documentation, then identify Linkedom’s existing Element serialization entry point. Compare the requested serializable shadow-root output with the example; done means supporting the described output for web components and shadow roots.