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/workletglobalscope.rs

@@ -27,9 +27,9 @@ use crate::dom::testworkletglobalscope::{TestWorkletGlobalScope, TestWorkletTask
 #[cfg(feature = "webgpu")]
 use crate::dom::webgpu::identityhub::IdentityHub;
 use crate::dom::worklet::WorkletExecutor;
+use crate::messaging::MainThreadScriptMsg;
 use crate::script_module::ScriptFetchOptions;
 use crate::script_runtime::{CanGc, JSContext};
-use crate::script_thread::MainThreadScriptMsg;
 
 #[dom_struct]
 /// <https://drafts.css-houdini.org/worklets/#workletglobalscope>
@@ -49,7 +49,7 @@ pub struct WorkletGlobalScope {
 
 impl WorkletGlobalScope {
     /// Create a new heap-allocated `WorkletGlobalScope`.
-    pub fn new(
+    pub(crate) fn new(
         scope_type: WorkletGlobalScopeType,
         runtime: &Runtime,
         pipeline_id: PipelineId,
@@ -76,7 +76,7 @@ impl WorkletGlobalScope {
     }
 
     /// Create a new stack-allocated `WorkletGlobalScope`.
-    pub fn new_inherited(
+    pub(crate) fn new_inherited(
         pipeline_id: PipelineId,
         base_url: ServoUrl,
         executor: WorkletExecutor,
@@ -172,7 +172,7 @@ impl WorkletGlobalScope {
 
 /// Resources required by workletglobalscopes
 #[derive(Clone)]
-pub struct WorkletGlobalScopeInit {
+pub(crate) struct WorkletGlobalScopeInit {
     /// Channel to the main script thread
     pub to_script_thread_sender: Sender<MainThreadScriptMsg>,
     /// Channel to a resource thread