libservo: Allow embedders to execute JavaScript scripts via the API (#35720)

This change adds a new `WebView` API `evaluate_javascript()`, which
allows embedders to
execute JavaScript code and wait for a reply asynchronously. Ongoing
script execution is
tracked by a libservo `JavaScriptEvaluator` struct, which maps an id to
the callback passed
to the `evaluate_javascript()` method. The id is used to track the
script and its execution
through the other parts of Servo.

Testing: This changes includes `WebView` unit tests.

---------

Signed-off-by: Narfinger <Narfinger@users.noreply.github.com>
Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Martin Robinson <mrobinson@igalia.com>

components/script/messaging.rs

@@ -91,6 +91,7 @@ impl MixedMessage {
                 #[cfg(feature = "webgpu")]
                 ScriptThreadMessage::SetWebGPUPort(..) => None,
                 ScriptThreadMessage::SetScrollStates(id, ..) => Some(*id),
+                ScriptThreadMessage::EvaluateJavaScript(id, _, _) => Some(*id),
             },
             MixedMessage::FromScript(inner_msg) => match inner_msg {
                 MainThreadScriptMsg::Common(CommonScriptMsg::Task(_, _, pipeline_id, _)) => {