script: Refactor channels in ScriptThread into receivers and senders (#34776)

Create two new data structures in the `script` crate to hold senders and
receiver:

- `ScriptThreadSenders`: holds all outgoing channels from the
  `ScriptThread` including a channel to the `ScriptThread` itself. The
  ultimate goal with this is to reduce duplication by giving a boxed
  version of this this to `Window`s.
- `ScriptThradReceivers`: holds all incoming channels to the
  `ScriptThread`. This isn't cloenable like the senders. This is used to
  abstract away `recv()` and `try_recv()` methods used to make the
  `ScriptThread` event loop easier to read.

In addition:

- The many duplicated `ScriptThread` self-senders for the `TaskManager`
  have been removed and, in general, a lot of boilerplate is removed as
  well.
- Visibilty of all methods affected by this change is changed to
  `pub(crate)` in order to take advantage of dead code detection. Some
  dead code produced from macros is removed.
- Some conversion code is refactord into implementations of the `From`
  trait.
- The names of channels uses a standard "sender" and "receiver" naming
  as well as trying to be descriptive of where they go in `ScriptThread`
  as well as `InitialScriptState`

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

components/script/dom/globalscope.rs

@@ -127,6 +127,7 @@ use crate::dom::webgpu::identityhub::IdentityHub;
 use crate::dom::window::Window;
 use crate::dom::workerglobalscope::WorkerGlobalScope;
 use crate::dom::workletglobalscope::WorkletGlobalScope;
+use crate::messaging::MainThreadScriptChan;
 use crate::microtask::{Microtask, MicrotaskQueue, UserMicrotask};
 use crate::network_listener::{NetworkListener, PreInvoke};
 use crate::realms::{enter_realm, AlreadyInRealm, InRealm};
@@ -134,7 +135,7 @@ use crate::script_module::{DynamicModuleList, ModuleScript, ModuleTree, ScriptFe
 use crate::script_runtime::{
     CanGc, CommonScriptMsg, JSContext as SafeJSContext, ScriptChan, ScriptPort, ThreadSafeJSContext,
 };
-use crate::script_thread::{with_script_thread, MainThreadScriptChan, ScriptThread};
+use crate::script_thread::{with_script_thread, ScriptThread};
 use crate::security_manager::CSPViolationReporter;
 use crate::task::TaskCanceller;
 use crate::task_source::dom_manipulation::DOMManipulationTaskSource;
@@ -2581,7 +2582,9 @@ impl GlobalScope {
     /// `ScriptChan` to send messages to the event loop of this global scope.
     pub fn script_chan(&self) -> Box<dyn ScriptChan + Send> {
         if let Some(window) = self.downcast::<Window>() {
-            return MainThreadScriptChan(window.main_thread_script_chan().clone()).clone();
+            return Box::new(
+                MainThreadScriptChan(window.main_thread_script_chan().clone()).clone(),
+            );
         }
         if let Some(worker) = self.downcast::<WorkerGlobalScope>() {
             return worker.script_chan();