refactor(core): JsRuntime is not a Future (#7855)

This commit rewrites deno_core::JsRuntime to not implement Future
trait.

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

cli/worker.rs

@@ -240,7 +240,7 @@ impl Future for Worker {
     // We always poll the inspector if it exists.
     let _ = inner.inspector.as_mut().map(|i| i.poll_unpin(cx));
     inner.waker.register(cx.waker());
-    inner.js_runtime.poll_unpin(cx)
+    inner.js_runtime.poll_event_loop(cx)
   }
 }