Support for breakpoint debugging (#371 / EW-7264)

This change places the InspectorService in a separate thread that
manages communication with over the websocket to the inspector.

The change also adds support for runMessageLoopOnPause() and
quitMessageLoopOnPause() to support breakpoints and debugger
break statements.

There is also refactoring of the CDP message handling code so it can
be called with or without the isolate lock held.

This requires workerd to run with the command-line flag -i to turn
on inspector support.

To try this out using samples/helloworld as an example:

1) Edit "samples/helloworld/worker.js" and add a debugger statement
   to the handle(request) method.
2) Open workerd in VSCode, select 'workerd with inspector enabled (dbg)'
   as the Run and Debug Target panel. Hit F5 to run and select
   `samples/helloworld/config.capnp` as the config to use.
3) Open devtools in Chrome using either:
   * https://devtools.devprod.cloudflare.dev/js_app?ws=localhost:9229/main
   * chrome:://inspect
4) On the command-line run, `curl http://localhost:8080/`
5) Devtools should break into the running worker.

Bug: #371
Test: manual
Test: existing internal ew tests do not break