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/worklet.rs

@@ -49,9 +49,10 @@ use crate::dom::workletglobalscope::{
     WorkletGlobalScope, WorkletGlobalScopeInit, WorkletGlobalScopeType, WorkletTask,
 };
 use crate::fetch::load_whole_resource;
+use crate::messaging::MainThreadScriptMsg;
 use crate::realms::InRealm;
 use crate::script_runtime::{CanGc, CommonScriptMsg, Runtime, ScriptThreadEventCategory};
-use crate::script_thread::{MainThreadScriptMsg, ScriptThread};
+use crate::script_thread::ScriptThread;
 use crate::task::TaskBox;
 use crate::task_source::TaskSourceName;
 
@@ -277,7 +278,7 @@ impl Drop for WorkletThreadPool {
 impl WorkletThreadPool {
     /// Create a new thread pool and spawn the threads.
     /// When the thread pool is dropped, the threads will be asked to quit.
-    pub fn spawn(global_init: WorkletGlobalScopeInit) -> WorkletThreadPool {
+    pub(crate) fn spawn(global_init: WorkletGlobalScopeInit) -> WorkletThreadPool {
         let primary_role = WorkletThreadRole::new(false, false);
         let hot_backup_role = WorkletThreadRole::new(true, false);
         let cold_backup_role = WorkletThreadRole::new(false, true);