refactor: Worker is not a Future (#7895)

This commit rewrites deno::Worker to not implement Future
trait.

Instead there are two separate methods:
- Worker::poll_event_loop() - does single tick of event loop
- Worker::run_event_loop() - runs event loop to completion

Additionally some cleanup to Worker's field visibility was done.

cli/ops/worker_host.rs

@@ -173,7 +173,8 @@ fn run_worker_thread(
     // TODO(bartlomieju): this thread should return result of event loop
     // that means that we should store JoinHandle to thread to ensure
     // that it actually terminates.
-    rt.block_on(worker).expect("Panic in event loop");
+    rt.block_on(worker.run_event_loop())
+      .expect("Panic in event loop");
     debug!("Worker thread shuts down {}", &name);
   })?;