feat: service worker rendering

[thomasfosterau]

closes #3498

⚠️ Work in Progress (feedback/suggestions/comments welcome) ⚠️

This PR is an attempt to introduce a minimally-viable version of rendering in Service Workers. It uses the API previously attempted in #5690 and which I’ve described in the discussion #13784.

Currently I am trying to implement rendering in service workers by duplicating and modifying the server runtime and then using a heavily refactored version of the Cloudflare adapter to expose a respond(event: FetchEvent): Promise<Response> function in $service-worker. The service-worker runtime delegates all server loads, actions and endpoints to the server, leaving it up to universal load functions to handle timeouts/connectivity/etc. (as they should already do, right?).

Some unresolved issues at this point:

• Does there need to be a hooks file for the service worker and server to share, e.g. hooks.ssr.{js,ts}? Maybe a hooks.worker.{js,ts}?
• How should the respond() function handle requests for assets?
• How much code could/should the server and service worker runtimes share?

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