Add workerd diagnostic

What does this merge request do and why?

This MR adds the Workerd diagnostic to find

Related to #2159

How to set up and validate locally

1. Run gdk start
2. Create a file named workerd.conf with:

   using Workerd = import "/workerd/workerd.capnp";
   const config :Workerd.Config = (
     services = [(name = "main", worker = .mainWorker),],
     sockets = [( name = "http", address = "*:8080", http = (), service = "main" ),]
   );
   const mainWorker :Workerd.Worker = (
     serviceWorkerScript = "addEventListener('fetch', event => {event.respondWith(new Response('Hello World'));});",
     compatibilityDate = "2023-02-28",
   );

   This will create a dummy workerd process that simulates a lingering workerd process.
3. Run gdk doctor
4. See the warning

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.