Use GenericChannel for script_chan (#38645)

Motivation: 
Using our GenericChannel abstraction allows us to optimize IPC in
single-process mode to just use cross-beam channel.
To keep the diff low, and get early feedback, this PR only tackles a
single channel, but the intention is to port all ipc channels to the
generic channel, which allows us to skip serializing and deserializing
messages in single process mode.

Based on: 
- https://github.com/servo/servo/pull/38638
- https://github.com/servo/servo/pull/38636

Testing: Covered by existing tests

---------

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

components/script/messaging.rs

@@ -8,6 +8,7 @@ use std::cell::RefCell;
 use std::option::Option;
 use std::result::Result;
 
+use base::generic_channel::GenericSender;
 use base::id::PipelineId;
 #[cfg(feature = "bluetooth")]
 use bluetooth_traits::BluetoothRequest;
@@ -331,7 +332,7 @@ pub(crate) struct ScriptThreadSenders {
 
     /// A [`Sender`] that sends messages to the `Constellation`.
     #[no_trace]
-    pub(crate) constellation_sender: IpcSender<ScriptThreadMessage>,
+    pub(crate) constellation_sender: GenericSender<ScriptThreadMessage>,
 
     /// A [`Sender`] that sends messages to the `Constellation` associated with
     /// particular pipelines.