We should have some form of support in SSR for events.

#2309 proposes an implementation. Questions (copied from that PR description):

• Can we have an event path of just the custom elements without materializing the built-ins that would be on the path with a full DOM structure? This seems ok if the only event patterns we support are for cross-custom-element communication, ie context.
• Where do elements install event listeners? I added a call to connectedCallback() in here, but that only doesn't throw because none of the test elements do anything with the DOM in connectedCallback(). That wouldn't be viable in general.
• Do we need to implement the capture phase?
• How could we implement the proper composed path? We would need some concept of slotted elements being in the custom element instance stack.