Combine some access to the thread local variable for script thread. (#38752)

This combines some access to the thread local variable for script
thread.

- We introduce a new UserInteractingScriptGuard which on drop handles
  the resetting of was_interacting to the previous value. Sometimes
throughout the code `ScriptThread::is_user_interacting` was reset to the
previous value while sometimes just set to false. This should
remove this footgun.
- This also reduces the amount of thread local access for
MutationObservers and task queue.

Testing: WPT tests should cover this.
Fixes: This addresses part of
https://github.com/servo/servo/issues/37969 but there is probably still
stuff to be done.

---------

Signed-off-by: Narfinger <Narfinger@users.noreply.github.com>
Signed-off-by: Josh Matthews <josh@joshmatthews.net>
Co-authored-by: Josh Matthews <josh@joshmatthews.net>

components/script/dom/html/htmlslotelement.rs

@@ -28,7 +28,6 @@ use crate::dom::document::Document;
 use crate::dom::element::{AttributeMutation, Element};
 use crate::dom::globalscope::GlobalScope;
 use crate::dom::html::htmlelement::HTMLElement;
-use crate::dom::mutationobserver::MutationObserver;
 use crate::dom::node::{BindContext, Node, NodeDamage, NodeTraits, ShadowIncluding, UnbindContext};
 use crate::dom::virtualmethods::VirtualMethods;
 use crate::script_runtime::CanGc;
@@ -357,7 +356,8 @@ impl HTMLSlotElement {
         ScriptThread::add_signal_slot(self);
 
         // Step 2. Queue a mutation observer microtask.
-        MutationObserver::queue_mutation_observer_microtask();
+        let mutation_observers = ScriptThread::mutation_observers();
+        mutation_observers.queue_mutation_observer_microtask(ScriptThread::microtask_queue());
     }
 
     pub(crate) fn remove_from_signal_slots(&self) {