Add direct script to embedder channel (#39039)

This PR **removes** `ScriptToConstellationMessage::ForwardToEmbedder`,
and replaces it with an explicit `ScriptToEmbedderChannel`. This new
channel is based on `GenericCallback` and in single-process mode will
directly send the message the to the embedder and wake it. In
multi-process mode, the message is routed via the ROUTER, since waking
is only possible from the same process currently. This means in
multi-process mode there are likely no direct perf benefits, since we
still need to hop the message over the ROUTER (instead of over the
constellation).
In single-process mode we can directly send the message to the embedder,
which should provide a noticable latency improvement in all cases where
script is blocked waiting on the embedder to reply.

This does not change the way the embedder receives messages - the
receiving end is unchanged.

## How was sending messages to the embedder working before?

1. Script wraps it's message to the embedder in
`ScriptToConstellationMessage::ForwardToEmbedder` and sends it to
constellation.
2. The [constellation event loop] receives the message in
[handle_request]
3. If deserialization fails, [an error is logged and the message is
ignored]
4. Since our message came from script, it is handle in
[handle_request_from_script]
5. The message is logged with trace log level
6. If the pipeline is closed, [a warning is logged and the message
ignored]
7. The wrapped `EmbedderMsg` [is forwarded to the embedder]. Sending the
message also invokes `wake()` on the embedder eventloop waker.

[constellation event loop]:
2e1b2e7260/components/constellation/constellation.rs (L755)

[handle request]:
2e1b2e7260/components/constellation/constellation.rs (L1182)

[an error is logged and the message is ignored]:
2e1b2e7260/components/constellation/constellation.rs (L1252)

[handle_request_from_script]:
https://github.com/servo/servo/blob/main/components/constellation/constellation.rs#L1590
 
[a warning is logged and the message ignored]:
2e1b2e7260/components/constellation/constellation.rs (L1599)

[is forwarded to the embedder]:
2e1b2e7260/components/constellation/constellation.rs (L1701)

Testing: Communication between Script and Embedder is extensive, so this
should be covered by existing tests.

Signed-off-by: Jonathan Schwender <schwenderjonathan@gmail.com>

components/script/dom/workerglobalscope.rs

@@ -90,6 +90,7 @@ pub(crate) fn prepare_workerscope_init(
         time_profiler_chan: global.time_profiler_chan().clone(),
         from_devtools_sender: devtools_sender,
         script_to_constellation_chan: global.script_to_constellation_chan().clone(),
+        script_to_embedder_chan: global.script_to_embedder_chan().clone(),
         worker_id: worker_id.unwrap_or_else(|| WorkerId(Uuid::new_v4())),
         pipeline_id: global.pipeline_id(),
         origin: global.origin().immutable().clone(),
@@ -185,6 +186,7 @@ impl WorkerGlobalScope {
                 init.mem_profiler_chan,
                 init.time_profiler_chan,
                 init.script_to_constellation_chan,
+                init.script_to_embedder_chan,
                 init.resource_threads,
                 MutableOrigin::new(init.origin),
                 init.creation_url,