script: Move TaskManager to GlobalScope (#34827)

This is a simplification of the internal `TaskQueue` API that moves the
`TaskManager` to the `GlobalScope` itself. In addition, the handling of
cancellers is moved to the `TaskManager` as well. This means that no
arguments other than the `task` are necessary for queueing tasks, which
makes the API a lot easier to use and cleaner.

`TaskSource` now also keeps a copy of the canceller with it, so that
they always know the proper way to cancel any tasks queued on them.

There is one complication here. The event loop `sender` for dedicated
workers is constantly changing as it is set to `None` when not handling
messages. This is because this sender keeps a handle to the main
thread's `Worker` object, preventing garbage collection while any
messages are still in flight or being handled. This change allows
setting the `sender` on the `TaskManager` to `None` to allow proper
garbabge collection.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/script/dom/htmldialogelement.rs

@@ -122,6 +122,6 @@ impl HTMLDialogElementMethods<crate::DomTypeHolder> for HTMLDialogElement {
         // Step 5
         win.task_manager()
             .dom_manipulation_task_source()
-            .queue_simple_event(target, atom!("close"), &win);
+            .queue_simple_event(target, atom!("close"));
     }
 }