Lit SSR DOM emulation

{% labs-disclaimer %}

When running in Node, Lit automatically imports and uses a set of DOM shims, and defines the customElements global. Only the minimal DOM interfaces needed to define and register components are implemented. These include a few key DOM classes and a roughly functioning CustomElementRegistry.

✅ signifies item is implemented to be functionally the same as in the browser.

<!-- TODO(augustinekim) Consider replacing emojis below with icons https://github.com/lit/lit.dev/pull/880#discussion_r944821511 -->

Property	Notes
Element	⚠️ Partial <table><tbody><tr><td>attributes</td><td>✅</td><tr><td>shadowRoot</td><td>⚠️ Returns {host: this} if attachShadow() was called with {mode: 'open'}</td><tr><td>setAttribute()</td><td>✅</td><tr><td>removeAttribute()</td><td>✅</td><tr><td>hasAttribute()</td><td>✅</td><tr><td>attachShadow()</td><td>⚠️ Returns {host: this}</td><tr><td>getAttribute()</td><td>✅</td></tr></tbody></table>
HTMLElement	⚠️ Empty class
CustomElementRegistry	<table><tbody><tr><td>define()</td><td>✅</td></tr><tr><td>get()</td><td>✅</td></tr></tbody></table>
customElements	Instance of CustomElementRegistry