script: add skeleton implementation of FontFace API (#35262)

This patch implements the `FontFace` interface, but with some caveats

1. The interface is only exposed on `Window`. Support for Workers will
   be handled in the future.
2. The concept of `css-connected` `FontFace` is not implemented, so
   `@font-face` rules in stylesheets will not be represented in the DOM.
3. The constructor only supports using `url()` strings as source
   and `ArrayBuffer` and `ArrayBufferView` are not supported yet.

A skeleton implementation of the `load` method of `FontFaceSet` is also
implemented in this patch. The intention is to support some web pages
that don't load without this method.

Signed-off-by: Mukilan Thiyagarajan <mukilan@igalia.com>

components/script/dom/eventsource.rs

@@ -594,7 +594,7 @@ impl EventSourceMethods<crate::DomTypeHolder> for EventSource {
             last_event_id: String::new(),
             resource_timing: ResourceFetchTiming::new(ResourceTimingType::Resource),
         };
-        let listener = NetworkListener {
+        let mut listener = NetworkListener {
             context: Arc::new(Mutex::new(context)),
             task_source: global.task_manager().networking_task_source().into(),
         };